Knowing chemical formula, one can calculate mass fraction chemical elements in the substance. element in the substance is denoted by the Greek. the letter "omega" - ω E / V and is calculated by the formula:
where k is the number of atoms of this element in a molecule.
What is the mass fraction of hydrogen and oxygen in water (H 2 O)?
Solution:
M r (H 2 O) = 2 * A r (H) + 1 * A r (O) = 2 * 1 + 1 * 16 = 18
2) We calculate the mass fraction of hydrogen in water:
3) We calculate the mass fraction of oxygen in water. Since the composition of water contains atoms of only two chemical elements, the mass fraction of oxygen will be equal to:
Rice. 1. Registration of the solution to problem 1
Calculate the mass fraction of elements in the substance H 3 PO 4.
1) Calculate the relative molecular weight of the substance:
M r (H 3 PO 4) = 3 * A r (H) + 1 * A r (R) + 4 * A r (O) = 3 * 1 + 1 * 31 + 4 * 16 = 98
2) We calculate the mass fraction of hydrogen in the substance:
3) We calculate the mass fraction of phosphorus in the substance:
4) We calculate the mass fraction of oxygen in the substance:
1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.А. Orzhekovsky et al. "Chemistry, grade 8" / P.А. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006.
2. Ushakova O.V. Chemistry workbook: grade 8: to the textbook by P.A. Orzhekovsky and others. "Chemistry. Grade 8 "/ О.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006. (p. 34-36)
3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005. (§15)
4. Encyclopedia for children. Volume 17. Chemistry / Chap. ed. by V.A. Volodin, led. scientific. ed. I. Leenson. - M .: Avanta +, 2003.
1. Unified collection of digital educational resources ().
2. Electronic version magazine "Chemistry and Life" ().
4. Video tutorial on the topic "Mass fraction of a chemical element in a substance" ().
Homework
1.p.78 No. 2 from the textbook "Chemistry: 8th grade" (PA Orzhekovsky, LM Meshcheryakova, LS Pontak. M .: AST: Astrel, 2005).
2. with. 34-36 No. 3,5 from Workbook in chemistry: grade 8: to the textbook by P.A. Orzhekovsky and others. "Chemistry. Grade 8 "/ О.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006.
Since the 17th century. chemistry has ceased to be a descriptive science. Chemical scientists began to make extensive use of the measurement of matter. The design of the balance, which makes it possible to determine the mass of the samples, has been improved more and more. For gaseous substances in addition to mass, volume and pressure were also measured. The use of quantitative measurements made it possible to understand the essence of chemical transformations, to determine the composition of complex substances.
As you already know, a complex substance contains two or more chemical elements. It is obvious that the mass of all matter is composed of the masses of its constituent elements. This means that the share of each element accounts for a certain part of the mass of the substance.
The mass fraction of an element is the ratio of the mass of this element in complex substance to the mass of the whole substance, expressed in fractions of a unit (or in percent):
Mass fraction of an element in a compound is denoted in Latin lowercase letter w("Double-ve") and shows the proportion (part of the mass) attributable to a given element in the total mass of the substance. This value can be expressed in fractions of a unit or as a percentage. Of course, the mass fraction of an element in a complex substance is always less than one (or less than 100%). After all, a part of a whole is always less than a whole, like an orange slice is the least of an orange.
For example, mercury oxide contains two elements - mercury and oxygen. When 50 g of this substance is heated, 46.3 g of mercury and 3.7 g of oxygen are obtained (Fig. 57). Let's calculate the mass fraction of mercury in a complex substance:
The mass fraction of oxygen in this substance can be calculated in two ways. By definition, the mass fraction of oxygen in mercury oxide is equal to the ratio of the mass of oxygen to the mass of the oxide:
Knowing that the sum of the mass fractions of elements in a substance is equal to one (100%), the mass fraction of oxygen can be calculated from the difference:
w(O) = 1 - 0.926 = 0.074,
w(O) = 100% - 92.6% = 7.4%.
In order to find the mass fractions of elements by the proposed method, it is necessary to carry out a complex and laborious chemical experiment by determining the mass of each element. If the formula of a complex substance is known, the same problem can be solved much easier.
To calculate the mass fraction of an element, its relative atomic mass must be multiplied by the number of atoms ( n) of this element in the formula and divided by the relative molecular weight of the substance:
For example, for water (fig. 58):
M r(H 2 O) = 1 2 + 16 = 18,
Objective 1.Calculate the mass fractions of elements in ammonia, the formula of which is NH 3 .
Given:
substance ammonia NH 3.
Find:
w(N), w(H).
Solution
1) Calculate the relative molecular weight of ammonia:
M r(NH 3) = A r(N) + 3 A r(H) = 14 + 3 1 = 17.
2) Find the mass fraction of nitrogen in the substance:
3) Let's calculate the mass fraction of hydrogen in ammonia:
w(H) = 1 - w(N) = 1 - 0.8235 = 0.1765, or 17.65%.
Answer. w(N) = 82.35%, w(H) = 17.65%.
Objective 2.Calculate the mass fractions of elements in sulfuric acid having the formula H 2 SO 4 .
Given:
sulfuric acid H 2 SO 4.
Find:
w(H), w(S), w(O).
Solution
1) Calculate the relative molecular weight of sulfuric acid:
M r(H 2 SO 4) = 2 A r(H) + A r(S) + 4 A r(O) = 2 1 + 32 + 4 16 = 98.
2) Find the mass fraction of hydrogen in the substance:
3) Let's calculate the mass fraction of sulfur in sulfuric acid:
4. Calculate the mass fraction of oxygen in the substance:
w(O) = 1 - ( w(H) + w(S)) = 1 - (0.0204 + 0.3265) = 0.6531, or 65.31%.
Answer. w(H) = 2.04%, w(S) = 32.65%, w(O) = 65.31%.
More often, chemists have to solve the inverse problem: by the mass fractions of elements, determine the formula of a complex substance. We will illustrate how such tasks are solved with one historical example.
From natural minerals- tenorite and cuprite - two compounds of copper with oxygen (oxides) were isolated. They differed from each other in color and mass fraction of elements. In black oxide, the mass fraction of copper was 80%, and the mass fraction of oxygen was 20%. In red copper oxide, the mass fractions of elements were 88.9% and 11.1%, respectively. What are the formulas of these complex substances? Let's carry out simple mathematical calculations.
Example 1. Calculation of the chemical formula of black copper oxide ( w(Cu) = 0.8 and w(O) = 0.2).
x, y- by the number of atoms of chemical elements in its composition: Сu x O y.
2) The ratio of the indices is equal to the ratio of the quotients from dividing the mass fraction of the element in the compound by the relative atomic mass of the element:
3) The resulting ratio must be reduced to the ratio of integers: the indices in the formula showing the number of atoms cannot be fractional. To do this, divide the resulting numbers by the smaller (i.e., any) of them:
The resulting formula is CuO.
Example 2. Calculation of the formula for red copper oxide by known mass fractions w(Cu) = 88.9% and w(O) = 11.1%.
Given:
w(Cu) = 88.9%, or 0.889,
w(O) = 11.1%, or 0.111.
Find:
Solution
1) Let us denote the oxide formula Cu x O y.
2) Find the ratio of the indices x and y:
3) We present the ratio of indices to the ratio of integers:
Answer... The compound formula is Cu 2 O.
Now let's complicate the task a little.
Objective 3.According to elemental analysis, the composition of the calcined bitter salt, which was still used by alchemists as a laxative, is as follows: the mass fraction of magnesium is 20.0%, the mass fraction of sulfur is 26.7%, the mass fraction of oxygen is 53.3%.
Given:
w(Mg) = 20.0%, or 0.2,
w(S) = 26.7%, or 0.267,
w(O) = 53.3%, or 0.533.
Find:
Solution
1) Let us denote the formula of a substance using the indices x, y, z: Mg x S y O z.
2) Find the ratio of the indices:
3) Determine the value of the indices x, y, z:
Answer. The formula of the substance is MgSO 4.
1. What is called the mass fraction of an element in a complex substance? How is this value calculated?
2.
Calculate the mass fractions of elements in substances: a) carbon dioxide CO 2;
b) calcium sulfide CaS; c) sodium nitrate NaNO 3; d) aluminum oxide Al 2 O 3.
3. In which of the nitrogen fertilizers the mass fraction of the nutrient nitrogen is the highest: a) ammonium chloride NH 4 Cl; b) ammonium sulfate (NH 4) 2 SO 4; c) urea (NH 2) 2 CO?
4. In the mineral pyrite, 7 g of iron contains 8 g of sulfur. Calculate the mass fractions of each element in this substance and determine its formula.
5. The mass fraction of nitrogen in one of its oxides is 30.43%, and the mass fraction of oxygen is 69.57%. Determine the oxide formula.
6. In the Middle Ages, a substance was isolated from the ash of a fire, which was called potash and was used to make soap. Mass fractions of elements in this substance: potassium - 56.6%, carbon - 8.7%, oxygen - 34.7%. Determine the potash formula.
§ 5.1 Chemical reactions. Equations chemical reactions
A chemical reaction is the transformation of some substances into others. However, such a definition needs one essential addition. V nuclear reactor or in an accelerator, some substances are also converted into others, but such transformations are not called chemical. What is the matter here? Nuclear reactions take place in a nuclear reactor. They consist in the fact that the nuclei of the elements when colliding with particles high energy(they can be neutrons, protons and nuclei of other elements) - they are broken into fragments, which are the nuclei of other elements. The fusion of nuclei among themselves is also possible. These new nuclei then receive electrons from environment and thus the formation of two or more new substances is completed. All these substances are some kind of elements Periodic table... Examples of nuclear reactions used to open new items are given in §4.4.
Unlike nuclear reactions, in chemical reactions kernels are not affected atoms. All changes occur only in external electronic shells... Torn apart alone chemical bonds and others are formed.
Chemical reactions are called phenomena in which some substances with a certain composition and properties are converted into other substances - with a different composition and other properties. Moreover, the composition atomic nuclei there is no change.
Consider a typical chemical reaction: combustion natural gas(methane) in atmospheric oxygen. Those of you who have a gas stove at home can observe this reaction in your kitchen on a daily basis. Let's write the reaction as shown in fig. 5-1.
Rice. 5-1. Methane CH 4 and oxygen O 2 react with each other to form carbon dioxide CO 2 and water H 2 O. In this case, the bonds between C and H are broken in the methane molecule, and in their place there are bonds of carbon with oxygen. Hydrogen atoms, previously belonging to methane, form bonds with oxygen. The figure clearly shows that for the successful implementation of the reaction to one the methane molecule must be taken two oxygen molecules.
It is not very convenient to write a chemical reaction using pictures of molecules. Therefore, abbreviated formulas of substances are used to record chemical reactions - as shown in the lower part of Fig. 5-1. Such a record is called chemical reaction equation.
The number of atoms of different elements on the left and right sides of the equation is the same. On the left side one carbon atom in the composition of the methane molecule (CH 4), and in the right - the same we find a carbon atom in the CO 2 molecule. We will definitely find all four hydrogen atoms from the left side of the equation in the right - in the composition of water molecules.
In the equation of a chemical reaction to equalize the number of identical atoms in different parts equations are used odds which are recorded front formulas of substances. Coefficients should not be confused with indices in chemical formulas.
Consider another reaction - the conversion of calcium oxide CaO (quicklime) into calcium hydroxide Ca (OH) 2 (slaked lime) under the action of water.
Rice. 5-2. Calcium oxide CaO attaches a water molecule H 2 O to form
calcium hydroxide Ca (OH) 2.
Unlike mathematical equations, the left and right sides cannot be swapped in the equations of chemical reactions. The substances on the left side of the chemical reaction equation are called reagents, and on the right - reaction products... If we make a permutation of the left and right sides in the equation from Fig. 5-2, then we get the equation completely different chemical reaction:
If the reaction between CaO and H 2 O (Fig. 5-2) begins spontaneously and proceeds with the release of a large number heat, then for the last reaction, where the reagent is Ca (OH) 2, strong heating is required.
Please note: instead of an equal sign in the chemical reaction equation, you can use an arrow. The arrow is convenient because it shows direction the course of the reaction.
We also add that reagents and products may not necessarily be molecules, but also atoms - if any element or elements in pure form... For example:
H 2 + CuO = Cu + H 2 O
There are several ways to classify chemical reactions, of which we will consider two.
According to the first of them, all chemical reactions are distinguished according to the criterion changes in the number of starting and final substances... Here you can find 4 types of chemical reactions:
Reactions CONNECTIONS,
Reactions DECOMPOSITION,
Reactions EXCHANGE,
Reactions SUBSTITUTIONS.
Let us give specific examples such reactions. To do this, we return to the equations for obtaining slaked lime and the equation for obtaining quicklime:
CaO + H 2 O = Ca (OH) 2
Ca (OH) 2 = CaO + H 2 O
These reactions are related to different types chemical reactions. The first reaction is a typical reaction. connections, since during its course two substances CaO and H 2 O combine into one: Ca (OH) 2.
The second reaction Ca (OH) 2 = CaO + H 2 O is a typical reaction decomposition: here one substance Ca (OH) 2 decomposes to form two others.
In reactions exchange the amount of reagents and products is usually the same. In such reactions, the initial substances exchange atoms with each other and even whole constituent parts their molecules. For example, when a CaBr 2 solution is poured into an HF solution, a precipitate forms. In solution, calcium and hydrogen ions exchange bromine and fluorine ions with each other. The reaction takes place only in one direction because calcium and fluorine ions bind to an insoluble compound CaF 2 and after that the "reverse exchange" of ions is no longer possible:
CaBr 2 + 2HF = CaF 2 ¯ + 2HBr
When the CaCl 2 and Na 2 CO 3 solutions are drained, a precipitate also precipitates, because calcium and sodium ions exchange with each other particles CO 3 2– and Cl - with the formation of an insoluble compound - calcium carbonate CaCO 3.
CaCl 2 + Na 2 CO 3 = CaCO 3 ¯ + 2NaCl
The arrow next to the reaction product indicates that this compound is insoluble and precipitated. Thus, the arrow can also be used to indicate the removal of a product from a chemical reaction in the form of a precipitate (¯) or gas (). For example:
Zn + 2HCl = H 2 + ZnCl 2
The latter reaction belongs to another type of chemical reaction - reactions substitutions... Zinc replaced hydrogen in its combination with chlorine (HCl). In this case, hydrogen is released in the form of a gas.
Substitution reactions may outwardly be similar to exchange reactions. The difference lies in the fact that atoms of some simple substances that replace the atoms of one of the elements in a complex substance. For example:
2NaBr + Cl 2 = 2NaCl + Br 2 - reaction substitutions;
on the left side of the equation there is a simple substance - a molecule of chlorine Cl 2, and on the right side there is a simple substance - a molecule of bromine Br 2.
In reactions exchange and reagents and products are complex substances. For example:
CaCl 2 + Na 2 CO 3 = CaCO 3 ¯ + 2NaCl - reaction exchange;
in this equation, reactants and products are complex substances.
The division of all chemical reactions into reactions of compound, decomposition, substitution and exchange is not the only one. There is another way of classification: on the basis of a change (or no change) in the oxidation states of reagents and products. On this basis, all reactions are divided into redox reactions and all others (not redox).
The reaction between Zn and HCl is not only a substitution reaction, but also redox reaction, because the oxidation states of the reacting substances change in it:
Zn 0 + 2H +1 Cl = H 2 0 + Zn +2 Cl 2 is a substitution reaction and at the same time a redox reaction.
Mass fraction of a substance is the ratio of the mass of a certain substance to the mass of a mixture or solution in which this substance is located. Expressed in fractions of a unit or as a percentage.
Instructions
1. The mass fraction of a substance is found by the formula: w = m (w) / m (cm), where w is the mass fraction of the substance, m (w) is the mass of the substance, m (cm) is the mass of the mixture. If the substance is dissolved, then the formula looks like this: w = m (w) / m (solution), where m (solution) is the mass of the solution. The mass of the solution, if necessary, is also allowed to be detected: m (solution) = m (c) + m (solution), where m (solution) is the mass of the solvent. If desired, the mass fraction can be multiplied by 100%.
2. If the value of the mass is not given in the condition of the problem, then it is allowed to calculate it with the support of several formulas, the values given in the condition will help to prefer the necessary one. The first formula for finding mass: m = V * p, where m is mass, V is volume, p is density. The further formula looks like this: m = n * M, where m is the mass, n is the number of the substance, M is the molar mass. The molar mass, in turn, is made up of the nuclear masses of the elements that make up the substance.
3. For better understanding of this material let's solve the problem. A mixture of copper and magnesium sawdust weighing 1.5 g was treated with an excess of sulfuric acid. As a result of the reaction, 0.56 L of hydrogen was released (typical data). Calculate the mass fraction of copper in the mixture. In this problem, a reaction takes place, we write down its equation. From 2 substances in excess of hydrochloric acid only magnesium interacts: Mg + 2HCl = MgCl2 + H2. In order to find the mass fraction of copper in the mixture, you need to substitute the values in following formula: w (Cu) = m (Cu) / m (cm). Given the mass of the mixture, we find the mass of copper: m (Cu) = m (cm) - m (Mg). We are looking for the mass of magnesium: m (Mg) = n (Mg) * M (Mg). The equation of the reaction will help to find the number of magnesium substance. We find the number of hydrogen substance: n = V / Vm = 0.56 / 22.4 = 0.025 mol. The equation shows that n (H2) = n (Mg) = 0.025 mol. We calculate the mass of magnesium, knowing that the molar mass of magnesium is 24 g / mol: m (Mg) = 0.025 * 24 = 0.6 g. We find the mass of copper: m (Cu) = 1.5 - 0.6 = 0.9 g It remains to calculate the mass fraction: w (Cu) = 0.9 / 1.5 = 0.6 or 60%.
Mass fraction shows in percent or in fractions the table of contents of a substance in a solution or an element in the composition of a substance. Knowing to calculate the mass fraction is beneficial not only in chemistry lessons, but also when you want to prepare a solution or mixture, say, for culinary purposes. Either change percentage, in the composition that you have.
Instructions
1. The mass fraction is calculated as the ratio of the mass of a given component to the total mass of the solution. To acquire the total in percent, you need to multiply the resulting quotient by 100. The formula looks like this:? = M (solute) / m (solution) ?,% =? * 100
2. Consider, for example, the direct and inverse problems: Let's say you dissolved 5 grams of table salt in 100 grams of water. What percentage of the solution did you receive? The solution is hefty primitive. You know the mass of the substance (table salt), the mass of the solution will be equal to the sum of the masses of water and salt. Thus, you should divide 5 g by 105 g and multiply the result of division by 100 - this will be the result: you will get a 4.7% solution. Now the reverse problem. You want to cook 200 g of 10% aqueous solution which is desirable. How much substance to take for dissolution? We operate in reverse order, the mass fraction, expressed as a percentage (10%), is divided by 100. We get 0.1. Now let's draw up a simple equation, where we denote the required number of substances by x and, consequently, the mass of the solution as 200 g + x. Our equation will look like this: 0.1 = x / 200g + x. When we solve it, we get that x equals approximately 22.2 g. The result is checked by solving the direct problem.
3. It is more difficult to find out what numbers of solutions of a known percentage you need to take for purchase a certain number solution with new specified qualities. Here it is required to compose and solve more closely a system of equations. In this system, the first equation is the expression of the famous mass of the resulting mixture, in terms of two unknown masses of the initial solutions. For example, if our goal is to get 150 g of a solution, the equation will have the form x + y = 150 g. The second equation is the mass of a solute equal to the sum of the same substance, in the composition of 2 mixed solutions. Say, if you want to have a 30% solution, and the solutions that you mix are 100%, that is, a pure substance, and 15%, then the second equation will look like: x + 0.15y = 45 g. for small, solve the equation system and find out how much substance you need to add to a 15% solution in order to get a 30% solution. Try it.
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To calculate number substances, find out its mass with the help of weights, express it in grams and divide by the molar mass that can be detected with the support of the periodic table. To determine the number substances gas under typical conditions, apply Avogadro's law. If the gas is in other conditions, measure the pressure, volume and temperature of the gas, and then calculate number substances in him.
You will need
- You will need a scale, thermometer, pressure gauge, ruler or tape measure, periodic table.
Instructions
1. Determining the number substances in a solid or liquid. Find the mass of the investigated body using scales, express it in grams. Determine which substances the body is composed, then with support periodic table Mendeleev find molar mass substances... To do this, find the elements that make up the molecule substances of which the body is made. Determine their nuclear masses from the table; if a fractional number is indicated in the table, round it to the nearest whole. Find the sum of the masses of all atoms in a molecule substances, get the molecular weight, which is numerically equal to the molar weight substances in grams per mole. Later, divide the previously measured mass by the molar mass. As a result, you will get number substances in moles (? = m / M).
2. Number substances gas under typical conditions. If the gas is in typical conditions (0 degrees Celsius and 760 mmHg), detect its volume. To do this, measure the volume of the room, cylinder or vessel where it is located, from the fact that the gas occupies each volume provided to it. In order to get its value, measure the geometric dimensions of the vessel, where it is located with the support of a tape measure and with the support of mathematical formulas, find its volume. A particularly classic case is a parallelepiped room. Measure its length, width and height in meters, then multiply them and get the volume of gas, which is in it in cubic meters. To discover number substances gas, divide the resulting volume by 0.0224 - the molar volume of gas under typical conditions.
3. Number substances gas with arbitrary parameters. Measure the gas pressure with a pressure gauge in pascals, its temperature in Kelvin, for which add 273 to the degrees Celsius in which the thermometer measures. Determine also the volume of gas in cubic meters. To discover number substances Divide the product of pressure and volume by the temperature and the number 8.31 (universal gas continuous),? = PV / (RT).
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Many liquids are solutions. These are, in particular, human blood, tea, coffee, sea water... The solution is based on a solute. There are tasks to find the mass fraction of this substance.
Instructions
1. Solutions are called homogeneous homogeneous systems that consist of 2 or more components. They are divided into three categories: - liquid solutions; - solid solutions; - gaseous solutions. Liquid solutions include, say, dilute sulfuric acid, solid - an alloy of iron and copper, and gaseous - all kinds of mixtures of gases. Regardless of the state of aggregation of the solution, it consists of a solvent and a solute. The most common solvent is water, which is used to dilute the substance. The composition of solutions is expressed in different ways, especially the value of the mass fraction of the solute is often used for this. The mass fraction is a dimensionless quantity, and it is equal to the ratio of the mass of the solute to the total mass of each solution:? In = mw / m The mass fraction is expressed as a percentage or decimal fractions. In order to calculate this parameter as a percentage, use the following formula: w (substance) = mw / m (solution) 100%. To find the same parameter in the form decimal do not multiply by 100%.
2. The mass of each solution is the sum of the masses of water and solute. Therefore, occasionally the formula indicated above is written in a slightly different way: Nitric acid consists of a solvent - water, and a solute - an acid. From this it follows that the mass of the solute is calculated in the following way:? In = mHNO3 / mHNO3 + mH2O
3. If the mass of a substance is unknown, but only the mass of water is given, then in this case the mass fraction is found according to a slightly different formula. When the volume of a dissolved substance is famous, its mass is found by the further formula: mw = V *? It follows from this that the mass fraction of a substance is calculated in the following way:? In = V *? / V *? + M (H2O)
4. Finding the mass fraction of a substance is repeatedly carried out for utilitarian purposes. For example, when bleaching a material, you need to know the concentration of perhydrol in a peroxide solution. In addition, an accurate calculation of the mass fraction is occasionally required in medical practice. In addition to formulas and an approximate calculation of the mass fraction, in medicine, experimental verification with the help of devices is also used, which makes it possible to reduce the likelihood of errors.
5. There are several physical processes during which the mass fraction of a substance and the composition of a solution change. The first of them, called evaporation, is the reverse process of dissolution of a substance in water. In this case, the solute remains, and the water is completely evaporated. In this case, the mass fraction cannot be measured - there is no solution. The opposite process is the dilution of a concentrated solution. The more it is diluted, the more strongly the mass fraction of the substance dissolved in it decreases. Concentration is a partial evaporation in which not every water evaporates, but only a part of it. At the same time, the mass fraction of the substance in the solution increases.
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What is mass fraction element? From the name itself, it is allowed to realize that this is a value indicating the ratio of the mass element, which is part of the substance, and the total mass of this substance. It is expressed in fractions of a unit: percent (hundredths), ppm (thousandths), etc. How is it allowed to calculate the mass of some element ?
Instructions
1. For clarity, consider the carbon that is well-known to everyone, without which there would be no organic matter. If carbon is a pure substance (say, diamond), then its mass share can be bravely taken as a unit or 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small numbers that they can be neglected. But in such modifications of carbon as coal or graphite, the content of impurities is quite high, and such ignoring is unacceptable.
2. If carbon is part of a difficult substance, you need to do it in a further way: write down the exact formula of the substance, after that, knowing the molar masses of each element included in its composition, calculate the exact molar mass of this substance (of course, taking into account the "index" of any element). Later this determine the mass share by dividing the total molar mass element per molar mass of the substance.
3. Let's say it is necessary to detect a mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. To simplify calculations, convert it to the form: С2Н4О2. The molar mass of this substance consists of the molar masses of the elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this substance is calculated as follows: 24/60 = 0.4.
4. If you need to calculate it in percentage, respectively, 0.4 * 100 = 40%. That is, every kilogram of acetic acid contains (approximately) 400 grams of carbon.
5. Of course, in absolutely the same way it is allowed to detect the mass fractions of all other elements. Let's say the mass fraction of oxygen in the same acetic acid is calculated as follows: 32/60 = 0.533 or approximately 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or approximately 6.7%.
6. To check the accuracy of the calculations, add the percentages of all elements: 40% (carbon) + 53.3% (oxygen) + 6.7% (hydrogen) = 100%. The account came together.
You have a two-hundred-liter barrel. You plan to fill it entirely with diesel fuel, which you use to heat your mini-boiler room. And how much will it weigh, filled with solarium? Now let's calculate.
You will need
- - table of specific gravity of substances;
- - knowledge to make the simplest mathematical calculations.
Instructions
1. In order to find the mass of a substance by its volume, use the formula for the specific density of the substance. P = m / v here p is the specific density of the substance; m is its mass; v is the occupied volume. We will consider the mass in grams, kilograms and tons. Volumes in cubic centimeters, decimeters and measures. And the specific density, respectively, in g / cm3, kg / dm3, kg / m3, t / m3.
2. It turns out, according to the terms of the problem, you have a two-hundred-liter barrel. This means: a barrel with a capacity of 2 m3. It is called two-hundred-liter, because water, with its specific gravity equal to one, contains 200 liters in such a barrel. You are worried about the mass. Consequently, bring it to the first place in the presented formula m = p * v On the right side of the formula, the value of p is unknown - the specific gravity of diesel fuel. Find it in the directory. It is even easier to enter a search query on the Internet “specific gravity of diesel fuel”.
3. Found: the density of summer diesel fuel at t = +200 C - 860 kg / m3. Substitute the values in the formula: m = 860 * 2 = 1720 (kg) 1 ton and 720 kg - this is how much 200 liters of summer diesel fuel weigh. Having hung the barrel in advance, it is allowed to calculate the total weight and estimate the capacity of the rack under the barrel with solarium.
4. V countryside it can be useful to preliminarily calculate the mass of firewood required by cubic capacity in order to determine the carrying capacity of the transport on which this firewood will be delivered. For example, you need at least 15 cubic meters for the winter. meters of birch firewood. Look in the reference books for the density of birch firewood. This is: 650 kg / m3. Calculate the mass by substituting the values in the same formula for specific gravity m = 650 * 15 = 9750 (kg) Now, based on the carrying capacity and capacity of the body, you can determine the type vehicle and the number of trips.
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Note!
Older people are more familiar with the concept of specific gravity. Specific gravity of a substance is the same as specific gravity.
The mass fraction of a substance shows its contents in a more difficult structure, say, in an alloy or mixture. If the total mass of a mixture or alloy is famous, then knowing the mass fractions of the constituent substances it is possible to detect their masses. To find the mass fraction of a substance, it is permissible to know its mass and the mass of each mixture. This value can be expressed in fractions or percentages.
You will need
- scales;
- periodic table of chemical elements;
- calculator.
Instructions
1. Determine the mass fraction of the substance that is in the mixture through the mass of the mixture and the substance itself. To do this, with the support of weights, determine the masses of the substances that make up the mixture or alloy. Then fold them up. Take the resulting mass as 100%. In order to find the mass fraction of a substance in a mixture, divide its mass m by the mass of the mixture M, and multiply the total by 100% (?% = (M / M)? 100%). Let's say that 20 g of sodium chloride is dissolved in 140 g of water. In order to find the mass fraction of salt, add the masses of these 2 substances M = 140 + 20 = 160 g. After that, find the mass fraction of the substance?% = (20/160)? 100% = 12.5%.
2. If you need to find the table of contents or the mass fraction of an element in a substance with a known formula, use the periodic table of chemical elements. On it, find the nuclear masses of the elements that make up the substance. If one element occurs several times in the formula, multiply its nuclear mass by this number and add up the results. This is the molecular weight of the substance. In order to find the mass fraction of any element in such a substance, divide its mass number in a given chemical formula M0 by the molecular mass of a given substance M. Multiply the total by 100% (?% = (M0 / M)? 100%).
3. Let's say, determine the mass fraction of chemical elements in copper sulfate. Copper sulfate (copper II sulfate), has the chemical formula CuSO4. The nuclear masses of the elements included in its composition are equal to Ar (Cu) = 64, Ar (S) = 32, Ar (O) = 16, the mass numbers of these elements will be equal to M0 (Cu) = 64, M0 (S) = 32, M0 (O) = 16 × 4 = 64, taking into account that the molecule contains 4 oxygen atoms. Calculate the molecular weight of the substance, it is equal to the sum of the mass numbers of the substances making up the molecule 64 + 32 + 64 = 160. Determine the mass fraction of copper (Cu) in the composition copper sulfate(?% = (64/160)? 100%) = 40%. According to the same thesis, it is allowed to determine the mass fractions of all elements in this substance. Mass fraction of sulfur (S)?% = (32/160)? 100% = 20%, oxygen (O)?% = (64/160)? 100% = 40%. Please note that the sum of all mass fractions of the substance must be 100%.
Mass fraction is the percentage of a component in a mixture or an element in a substance. It is not only schoolchildren and students who face the problems of calculating the mass fraction. The knowledge to calculate the percentage concentration of a substance finds absolutely utilitarian use in real life- where the preparation of solutions is required - from construction to cooking.
You will need
- - Mendeleev table;
- - formulas for calculating the mass fraction.
Instructions
1. Calculate the mass share a-priory. Because the mass of a substance is composed of the masses of the elements that make it up, then on share any constituent element is brought to a certain part of the mass of the substance. The mass fraction of the solution is equal to the ratio of the mass of the solute to the mass of each solution.
2. The mass of the solution is equal to the sum of the masses of the solvent (traditionally water) and the substance. The mass fraction of the mixture is equal to the ratio of the mass of the substance to the mass of the mixture containing the substance. Multiply the resulting total by 100%.
3. Detect massive share output with the support of the formula? = md / mp, where mp and md are the values of the assumed and actual obtained yield of the substance (mass), respectively. Calculate the assumed mass from the reaction equation using the formula m = nM, where n is the chemical number of the substance, M is the molar mass of the substance (the sum of the nuclear masses of all the elements included in the substance), or by the formula m = V?, Where V is the volume of the substance, ? - its density. The number of the substance, in turn, if necessary, replace with the formula n = V / Vm, or also find from the reaction equation.
4. Massive share calculate the element of difficult substance with the help of the periodic table. Add up the nuclear masses of all the elements that make up the substance, multiplying by indices if necessary. This will give you the molar mass of the substance. Find the molar mass of an element from the periodic table. Calculate the mass share by dividing the molar mass of the element by the molar mass of the substance. Multiply by 100%.
Helpful advice
Pay attention to physical process, the one that takes place. When evaporating, do not calculate the mass fraction, because there is no solution (water or any other liquid). Do not forget that during concentration, on the contrary, called partial evaporation, the mass fraction of the substance increases. If you dilute concentrated solution, the mass fraction decreases.
The mass fraction of any component in a substance shows which part of the total mass is brought into the atoms of this particular element. Applying the chemical formula of a substance and the periodic table of Mendeleev, it is allowed to determine the mass fraction of all of the elements included in the formula. The resulting value is expressed as a regular fraction or as a percentage.
Instructions
1. If you want to determine the mass fraction of each element that composes it by a chemical formula, start by calculating the number of atoms, which is brought to all of the elements. Let's say the chemical formula of ethanol is written as follows: CH? -CH? -OH. And the chemical formula of dimethyl ether is CH? -O-CH ?. The number of oxygen (O) atoms in any of the formulas is equal to one, carbon (C) - two, hydrogen (H) - six. Please note that this various substances, because the identical number of atoms of the whole element in their molecules is located differently. Nevertheless, the mass fractions of the whole element in dimethyl ether and ethanol will be identical.
2. Using the periodic table, determine the nuclear mass of each element in the chemical formula. Multiply this number by the number of atoms of each element calculated in the previous step. In the example used above, the formula each contains one oxygen atom, and its atomic mass from the table is 15.9994. There are two carbon atoms in the formula, its atomic mass is 12.0108, which means that the total weight of the atoms will be 12.0108 * 2 = 24.0216. For hydrogen, these numbers are 6, 1.00795 and 1.00795 * 6 = 6.0477, respectively.
3. Determine the total atomic mass of the entire molecule of the substance - add the numbers obtained in the previous step. For dimethyl ether and ethanol, this value should be equal to 15.9994 + 24.0216 + 6.0477 = 46.0687.
4. If the total is required to be obtained in fractions of one, make up an individual fraction for each element in the formula. Its numerator should contain the value calculated for this element in the second step, and put the number from the third step in the denominator of the entire fraction. The resulting ordinary fraction can be rounded to the required degree of accuracy. In the example used above, the mass fraction of oxygen is 15.9994 / 46.0687 × 16/46 = 8/23, carbon - 24.0216 / 46.0687 × 24/46 = 12/23, hydrogen - 6.0477 / 46, 0687? 6/46 = 3/23.
5. To obtain the total in percentage, convert the resulting ordinary fractions into decimal format and increase it by a factor of one hundred. In the example used, the mass fraction of oxygen in percent is expressed by the number 8/23 * 100 × 34.8%, carbon - 12/23 * 100 × 52.2%, hydrogen - 3/23 * 100 × 13.0%.
Related Videos
Note!
The mass fraction cannot be more than one or, if it is expressed as a percentage, more than 100%.
Since the 17th century. chemistry has ceased to be a descriptive science. Chemical scientists began to widely use methods for measuring various parameters of a substance. The design of balances has been improved more and more, allowing to determine the masses of samples for gaseous substances, in addition to the mass, the volume and pressure were also measured. The use of quantitative measurements made it possible to understand the essence of chemical transformations, to determine the composition of complex substances.
As you already know, a complex substance contains two or more chemical element... It is obvious that the mass of all matter is composed of the masses of its constituent elements. This means that the share of each element accounts for a certain part of the mass of the substance.
The mass fraction of an element in a substance is denoted by the Latin lowercase letter w (double-ve) and shows the proportion (part of the mass) attributable to this element in the total mass of the substance. This value can be expressed in fractions of a unit or as a percentage (Fig. 69). Of course, the mass fraction of an element in a complex substance is always less than one (or less than 100%). After all, a part of a whole is always less than a whole, like an orange slice is the least of an orange.
Rice. 69.
Diagram elemental composition mercury oxide
For example, mercury oxide HgO contains two elements - mercury and oxygen. When 50 g of this substance is heated, 46.3 g of mercury and 3.7 g of oxygen are obtained. Let's calculate the mass fraction of mercury in a complex substance:
The mass fraction of oxygen in this substance can be calculated in two ways. By definition, the mass fraction of oxygen in mercury oxide is equal to the ratio of the mass of oxygen to the mass of mercury oxide:
Knowing that the sum of the mass fractions of elements in a substance is equal to one (100%), the mass fraction of oxygen can be calculated from the difference:
In order to find the mass fractions of elements by the proposed method, it is necessary to carry out a complex and laborious chemical experiment to determine the mass of each element. If the formula of a complex substance is known, the same problem can be solved much easier.
To calculate the mass fraction of an element, its relative atomic mass must be multiplied by the number of atoms of a given element in the formula and divided by the relative molecular weight of the substance.
For example, for water (fig. 70):
Let's practice solving problems for calculating the mass fractions of elements in complex substances.
Task 1. Calculate the mass fractions of elements in ammonia, the formula of which is NH 3.
Problem 2. Calculate the mass fractions of elements in sulfuric acid having the formula H 2 SO 4.
More often, chemists have to solve the inverse problem: by the mass fractions of elements, determine the formula of a complex substance.
We will illustrate how such tasks are solved with one historical example.
Problem 3. From natural minerals - tenorite and cuprite (Fig. 71), two compounds of copper with oxygen (oxides) were isolated. They differed from each other in color and mass fraction of elements. In the black oxide (Fig. 72), isolated from tenorite, the mass fraction of copper was 80%, and the mass fraction of oxygen was 20%. In red copper oxide isolated from cuprite, the mass fractions of elements were 88.9% and 11.1%, respectively. What are the formulas of these complex substances? Let's solve these two simple tasks.
Rice. 71. Mineral cuprite
Rice. 72. Black copper oxide isolated from the mineral tenorite
3. The resulting ratio must be reduced to the values of whole numbers: after all, the indices in the formula showing the number of atoms cannot be fractional. To do this, the resulting numbers must be divided by the smaller of them (in our case, they are equal).
Now let's complicate the task a little.
Problem 4. According to elemental analysis data, the calcined bitter salt has the following composition: mass fraction of magnesium 20.0%, mass fraction of sulfur - 26.7%, mass fraction of oxygen - 53.3%.
Questions and tasks
- What is called the mass fraction of an element in a complex substance? How is this value calculated?
- Calculate the mass fractions of elements in substances: a) carbon dioxide CO 2; b) calcium sulfide CaS; c) sodium nitrate NaNO 3; d) aluminum oxide A1 2 O 3.
- In which of the nitrogen fertilizers the mass fraction of the nutrient nitrogen is the highest: a) ammonium chloride NH 4 C1; b) ammonium sulfate (NH 4) 2 SO 4; c) urea (NH 2) 2 CO?
- In the mineral pyrite, 7 g of iron contains 8 g of sulfur. Calculate the mass fractions of each element in this substance and determine its formula.
- The mass fraction of nitrogen in one of its oxides is 30.43%, and the mass fraction of oxygen is 69.57%. Determine the oxide formula.
- In the Middle Ages, a substance was isolated from the ash of a fire, which was called potash and was used to make soap. The mass fractions of elements in this substance are: potassium - 56.6%, carbon - 8.7%, oxygen - 34.7%. Determine the potash formula.
