Chemistry - definitely interesting science... Despite all its complexity, it allows us to better understand the nature of the world around us. And moreover - at least basic knowledge in this subject seriously helps in Everyday life... For example, the definition mass fraction substances in a multicomponent system, that is, the ratio of the mass of any component to the total mass of the entire mixture.
Necessary:
- calculator;
- scales (if you first need to determine the masses of all components of the mixture);
- periodic table of elements of Mendeleev.
Instructions:
- So, it became necessary for you to determine the mass fraction of a substance. Where to begin? First of all, it depends on the specific task and on the tools at hand for work. But in any case, in order to determine the content of a component in a mixture, you need to know its mass and the total mass of the mixture. This can be done either on the basis of known data or on the basis of your own research. To do this, you need to weigh the added component on a laboratory balance. After the mixture is cooked, weigh it also.
- Write down the mass of the required substance as “ m«, total mass systems, put under the designation " M". In this case, the formula for the mass fraction of the substance will take next view:W = (m / M) * 100. The result is recorded as a percentage.
- Example: calculate the mass fraction of 15 grams of sodium chloride dissolved in 115 g of water... Solution: the total mass of the solution is determined by the formula M = m in + m c, where m in- the mass of water, m c- the mass of table salt. From simple calculations, you can determine that the total mass of the solution is 130 gram... According to the above definition formula, we obtain that the content of sodium chloride in the solution will be equal to W = (15/130) * 100 = 12%.
- A more particular situation is the need to determine mass fraction chemical element in substance ... It is defined in the same way. The main principle the calculation will remain the same, only instead of the mass of the mixture and a specific component, you have to deal with the molecular masses of chemical elements.
- All the information you need can be found in the periodic table of Mendeleev. Break down the chemical formula of the substance into its main components. Using the periodic table, determine the mass of each element. Summing them up, you get the molecular weight of your substance ( M). Similarly to the previous case, the mass fraction of a substance, or to be more precise, of an element, will be determined by the ratio of its mass to its molecular mass. The formula will take the following form W = (m a / M) * 100. Where m a- the atomic mass of the element, M is the molecular weight of the substance.
- Let's consider this case with a specific example. Example: determine the mass fraction of potassium in potash... Potash is potassium carbonate. Its formula K 2 CO 3... Atomic mass of potassium - 39 , carbon - 12 , oxygen - 16 ... The molecular weight of the carbonate will be determined as follows - M = 2m K + m C + 2m O = 2 * 39 + 12 + 2 * 16 = 122... The potassium carbonate molecule contains two potassium atoms with an atomic mass equal to 39 ... The mass fraction of potassium in the substance will be determined by the formula W = (2m K / M) * 100 = (2 * 39/122) * 100 = 63.93%.
1. Fill in the blanks in the sentences.
a) In mathematics, "share" is the relationship of a part to a whole. To calculate the mass fraction of an element, you need to multiply its relative atomic mass by the number of atoms of this element in the formula and divided by the relative molecular weight of the substance.
b) The sum of the mass fractions of all the elements that make up the substance is 1 or 100%.
2. Write down mathematical formulas for finding the mass fractions of elements if:
a) the formula of the substance is P 2 O 5, M r = 2 * 31 + 5 * 16 = 142
w (P) = 2 * 31/132 * 100% = 44%
w (O) = 5 * 16/142 * 100% = 56% or w (O) = 100-44 = 56.
b) the substance formula - A x B y
w (A) = Ar (A) * x / Mr (AxBy) * 100%
w (B) = Ar (B) * y / Mr (AxBy) * 100%
3. Calculate the mass fraction of elements:
a) in methane (CH 4)
b) in sodium carbonate (Na 2 CO 3)
4. Compare the mass fractions of the indicated elements in substances and put a sign<, >or =:
5. In the combination of silicon with hydrogen, the mass fraction of silicon is 87.5%, hydrogen is 12.5%. The relative molecular weight of the substance is 32. Determine the formula of this compound.
6. Mass fractions of elements in the compound are shown in the diagram:
Determine the formula of this substance if it is known that its relative molecular weight is 100.
7. Ethylene is a natural stimulator of fruit ripening: its accumulation in fruits accelerates their ripening. The earlier the accumulation of ethylene begins, the earlier the fruits ripen. Therefore, ethylene is used to artificially accelerate the ripening of fruits. Derive the formula for ethylene if it is known that the mass fraction of carbon is 85.7%, and the mass fraction of hydrogen is 14.3%. The relative molecular weight of this substance is 28.
8. Derive the chemical formula of a substance, if it is known that
a) w (Ca) = 36%, w (Cl) = 64%
b) w (Na) 29.1%, w (S) = 40.5%, w (O) = 30.4%.
9. Lapis has antimicrobial properties. Previously, it was used to cauterize warts. In small concentrations, it acts as an anti-inflammatory and astringent, but can cause burns. Derive the formula for lapis if it is known that it contains 63.53% silver, 8.24% nitrogen, 28.23% oxygen.
Instructions
Determine the chemical form of the substance, the mass fractions of the elements of which you want to find. Take periodic system Mendeleev and find in it the cells of the elements corresponding to the atoms that make up the molecule of the given substance. In the cell, find the mass number of each such element... If the found value is a mass number element fractional, round to the nearest.
In the case when atoms of the same type are found in a molecule several times, multiply their atomic mass by this number. Add the masses of all the elements that make up the molecule to get the value in atomic mass units. For example, if you need to find the mass of a salt molecule, which is sulfate (Na2SO4), determines the atomic mass of sodium Ar (Na) = 23, sulfur Ar (S) = 32 and Ar (O) = 16. Since the molecule contains 2 sodium, then for it take the value 23 * 2 = 46, and, which has 4 atoms, 16 * 4 = 64. Then the mass of the molecule will be sodium sulfate will be Мr (Na2SO4) = 46 + 32 + 64 = 142.
To calculate the mass fractions of the elements that make up the molecule of a given substance, find the ratio of the masses of the atoms in the molecule of the substance to the mass of the molecule, and multiply the result by 100%. For example, if we consider sodium sulfate Na2SO4, calculate the mass fraction of its elements as follows: - the mass fraction of sodium will be ω (Na) = 23 2 100% / 142 = 32.4%;
- the mass fraction of sulfur will be ω (S) = 32 100% / 142 = 22.5%;
- the mass fraction of oxygen will be ω (O) = 16 4 100% / 142 = 45.1%.
Mass fractions show the relative elements in a given molecule of a substance. Check the correctness of the calculation by adding the mass fractions of the substance. Their amount should be 100%. In the example under consideration, 32.4% + 22.5% + 45.1% = 100%, the calculation is made.
Perhaps it is impossible to find an element as essential for life as oxygen. If a person can live without food for several weeks, without water for several days, then without oxygen - only a few minutes. This substance is widely used in different areas industry, including chemical, as well as a component rocket fuel(oxidizing agent).
Instructions
It is often necessary to determine the mass of oxygen in a closed volume, or as a result chemical reaction... For example: 20 grams of permanganate was subjected to thermal decomposition, the reaction went to the end. How many grams of oxygen were released during this?
First of all, remember that potassium - aka - has the chemical formula KMnO4. When heated, it decomposes, forming potassium manganate - K2MnO4, the main one - MnO2, and O2. Having written down the reaction equation, and choosing the coefficients, you get:
2KMnO4 = K2MnO4 + MnO2 + O2
Considering that the approximate molecular weight of two molecules of potassium permanganate is 316, and the molecular weight of an oxygen molecule, respectively, 32, by solving the proportion, calculate:
20 * 32 /316 = 2,02
That is, when thermally decomposing 20 grams of potassium permanganate, approximately 2.02 grams of oxygen is obtained. (Or roughly 2 grams).
Or, for example, it is required to determine the mass of oxygen in a closed volume, if its temperature and pressure are known. This is where the universal Mendeleev-Clapeyron equation comes to the rescue, or, in other words, the “ideal gas equation of state”. It looks like this:
PVm = MRT
P - gas pressure,
V is its volume,
m is its molar mass,
M - mass,
R - universal gas constant,
T is the temperature.
You see that the required value, that is, the mass of the gas (oxygen), after bringing all the initial data into one system of units (pressure -, temperature - in Kelvin, etc.), can be easily calculated by the formula:
Of course, real oxygen is not the ideal gas for which this equation was introduced. But at pressures and temperatures close to, the deviations of the calculated values from the actual ones are so insignificant that they can be safely neglected.
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What is mass fraction element? From the name itself, you can understand 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 can you calculate the mass of any element?
Instructions
For clarity, consider the well-known carbon to all, without which there would not be. If carbon is a substance (for example), then its mass share can be safely taken as a unit or 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small quantities that they can be neglected. But in such modifications of carbon as or, the content of impurities is quite high, and neglect is unacceptable.
If carbon is part of a complex substance, you must proceed as follows: write down the exact formula of the substance, then, 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 each element). After that, determine the mass share by dividing the total molar mass element per molar mass of the substance.
For example, you need to find a mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. For ease of calculations, convert it to the form: С2Н4О2. The molar mass of this substance consists of molar masses elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this substance is calculated as follows: 24/60 = 0.4.
If you need to calculate it in percentage, respectively, 0.4 * 100 = 40%. That is, each acetic acid contains (approximately) 400 grams of carbon.
Of course, in a completely analogous way, you can find the mass fractions of all other elements. For example, the mass in the same acetic acid is calculated as follows: 32/60 = 0.533 or about 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or about 6.7%.
Sources:
- mass fractions of elements
A chemical formula is a notation made using generally accepted symbols that characterizes the composition of a molecule of a substance. For example, the formula for the well-known sulfuric acid is H2SO4. It can easily be seen that each sulfuric acid molecule contains two hydrogen atoms, four oxygen atoms and one atom. It must be understood that this is only an empirical formula; it characterizes the composition of a molecule, but not its "structure", that is, the arrangement of atoms relative to each other.
You will need
- - Mendeleev table.
Instructions
First, find out the elements, the composition of the substance, and theirs. For example: what will nitric oxide be? It is obvious that the composition of the molecule of this two elements: nitrogen and. Both of them are gases, that is, they are pronounced. So what is the valency of nitrogen and oxygen in this compound?
Remember very important rule: non-metals have higher and lower valence. The highest corresponds to the group number (in in this case, 6 for oxygen and 5 for nitrogen), and the lowest is the difference between 8 and the group number (that is, the lowest valence for nitrogen is 3, and for oxygen - 2). The only exception to this rule is fluorine, which in all of its exhibits one valency equal to 1.
So what valency - the highest or the lowest - do nitrogen and oxygen have? One more rule: in compounds of two elements, the one that is located to the right and above in the periodic table shows the lowest valency. It is quite obvious that in your case it is oxygen. Therefore, in combination with nitrogen, oxygen has a valency equal to 2. Accordingly, nitrogen in this compound has a higher valency equal to 5.
Now remember the valency itself: this is the ability of an atom of an element to attach to itself a certain number of atoms of another element. Each nitrogen atom in this compound "" 5 oxygen atoms, and each oxygen atom - 2 nitrogen atoms. What is nitrogen? That is, what indexes does each element have?
One more rule will help to answer this question: the sum of the valencies of the elements included in the compound must be equal! What is the least common multiple for 2 and 5? Naturally, 10! Dividing it by the values of the valences of nitrogen and oxygen, you will find the indices and the final formula compounds: N2O5.
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The mass fraction of a substance shows its content in a more complex structure, for example, in an alloy or mixture. If the total mass of the mixture or alloy is known, then knowing the mass fractions of the constituent substances, you can find their masses. You can find the mass fraction of a substance by knowing its mass and the mass of the entire mixture. This value can be expressed in fractions or percentages.
You will need
- scales;
- periodic table of chemical elements;
- calculator.
Instructions
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, using the balance, determine the masses that make up the mixture or. Then fold them up. Take the resulting mass as 100%. 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 result by 100% (ω% = (m / M) ∙ 100%). For example, 20 g of sodium chloride is dissolved in 140 g of water. To find the mass fraction of salt, add the masses of these two substances M = 140 + 20 = 160 g. Then find the mass fraction of the substance ω% = (20/160) ∙ 100% = 12.5%.
If you want to find or the mass fraction of an element in a substance with known formula, use periodic table elements. Use it to find the atomic masses of the elements that are in the substance. If one is in the formula multiple times, multiply its atomic mass by that number and add up the results. This is the molecular weight of the substance. 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 this substance M. Multiply the result by 100% (ω% = (M0 / M) ∙ 100%).
You will need
- You need to determine which option your task belongs to. In the case of the first option, you will need the periodic table. In the case of the second, you need to know that the solution consists of two components: a solute and a solvent. And the mass of the solution is equal to the masses of these two components.
Instructions
In the case of the first variant of the problem:
According to Mendeleev, we find the molar mass of a substance. The molar sum of the atomic masses that make up the substance.
For example, the molar mass (Mr) of calcium hydroxide Ca (OH) 2: Mr (Ca (OH) 2) = Ar (Ca) + (Ar (O) + Ar (H)) * 2 = 40 + (16 + 1) * 2 = 74.
If there is no measuring container to pour water into, calculate the volume of the container in which it is located. The volume is always equal to the product of the base area by the height, and usually there are no problems with vessels of a stable shape. Volume water the bank will equal to the area a round base to a height filled with water. By multiplying the density? by volume water V, you get the mass water m: m =? * V.
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note
You can also determine the mass knowing the amount of water and its molar mass. The molar mass of water is 18, since it consists of the molar masses of 2 hydrogen atoms and 1 oxygen atom. MH2O = 2MH + MO = 21 + 16 = 18 (g / mol). m = n * M, where m is the mass of water, n is the amount, M is the molar mass.
What is mass fraction element? From the name itself, you can understand 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 can you calculate the mass of any element?
Instructions
For clarity, consider the well-known carbon to all, without which there would not be. If carbon is a substance (for example), then its mass share can be safely taken as a unit or 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small quantities that they can be neglected. But in such modifications of carbon as or, the content of impurities is quite high, and neglect is unacceptable.
If carbon is part of a complex substance, you must proceed as follows: write down the exact formula of the substance, then, 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 each element). After that, determine the mass share by dividing the total molar mass element per molar mass of the substance.
For example, you need to find a mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. For ease of 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.
If you need to calculate it as a percentage, respectively, 0.4 * 100 = 40%. That is, each acetic acid contains (approximately) 400 grams of carbon.
Of course, in a completely analogous way, you can find the mass fractions of all other elements. For example, the mass in the same acetic acid is calculated as follows: 32/60 = 0.533 or about 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or about 6.7%.
Sources:
- mass fractions of elements
The mass fraction of a substance shows its content in a more complex structure, for example, in an alloy or mixture. If the total mass of the mixture or alloy is known, then knowing the mass fractions of the constituent substances, you can find their masses. You can find the mass fraction of a substance by knowing its mass and the mass of the entire mixture. This value can be expressed in fractions or percentages.
>>
Mass fraction of an element in a complex substance
The material in the paragraph will help you:
> find out what the mass fraction of an element in a compound is, and determine its value;
> calculate the mass of the element in a certain mass of the compound, based on the mass fraction of the element;
> correctly formulate the solution of chemical problems.
Each complex substance (chemical compound) is formed by several elements. Knowing the content of elements in a compound is necessary for it effective use... For example, the best nitrogen fertilizer is the one that contains the largest number Nitrogen (this element is necessary for plants). The quality of the metal ore is assessed in a similar way, determining how much it “ rich»Onto a metal element.
Content element in conjunction characterize it mass fraction th. This value is denoted Latin letter w ("double-ve").
Let us derive a formula for calculating the mass fraction of an element in a compound by known to the masses connections and element. Let us denote the mass fraction of the element by the letter x. Taking into account that the mass of the compound is a whole, and the mass of an element is a part of the whole, we make up the proportion:
Note that the masses of the element and the compound must be taken in the same units of measurement (for example, in grams).
It is interesting
In two Sulfur compounds - SO 2 and MoS 3 - the mass fractions of the elements are the same and amount to 0.5 (or 50%) each.
Mass fraction has no dimension. It is often expressed as a percentage. In this case formula takes the following form:
It is obvious that the sum of the mass fractions of all elements in the compound is equal to 1 (or 100%).
Here are some examples of how to solve computational problems. The condition of the problem and its solution are formulated in this way. A sheet of notebook or chalkboard divide vertical line into two unequal parts. In the left, smaller, part, the condition of the problem is written in abbreviated form, a horizontal line is drawn and under it indicate what needs to be found or calculated. On the right side, they write down mathematical formulas, an explanation, calculations and an answer.
80 g of the compound contains 32 g Oxygena... Calculate the mass fraction of Oxygen in the compound.
The mass fraction of an element in a compound is also calculated using the chemical formula of the compound. Since the masses of atoms and molecules proportional to the relative atomic and molecular weights, then
where N (E) is the number of atoms of an element in the compound formula.
From a known mass fraction of an element, it is possible to calculate the mass of the element, which is contained in a certain mass of the compound. From the mathematical formula for the mass fraction of the element follows:
m (E) = w (E) m (compounds).
What mass of Nitrogen is contained in ammonium nitrate (nitrogen fertilizer) weighing 1 kg, if the mass fraction of this element in the compound is 0.35?
The term "mass fraction" is used to characterize the quantitative composition of mixtures of substances. The corresponding mathematical formula looks like this:
conclusions
The mass fraction of an element in a compound is the ratio of the mass of an element to the corresponding mass of a compound.
The mass fraction of an element in a compound is calculated by the known masses of the element and the compound or by its chemical formula.
?
92. How to calculate the mass fraction of an element in a compound if you know: a) the mass of the element and the corresponding mass of the compound; b) the chemical formula of the compound?
93. 20 g of substance contains 16 g of Bromine. Find the mass fraction of this element in a substance by expressing it ordinary shot, decimal and percentage.
94. Calculate (preferably orally) the mass fractions of elements in compounds with the following formulas: SO 2, LiH, CrO 3.
95. Comparing the formulas of substances, as well as the values of the relative atomic masses, determine in which of the substances of each pair the mass fraction of the first element in the formula is greater:
a) N 2 O, NO; b) CO, CO 2; c) B 2 O 3, B 2 S 3.
96. Perform the necessary calculations for acetic acid CH 3 COOH and glycerol C 3 H 5 (OH) 3 and fill in the table:
| C x H y O z | M r (C x H y O z) | w (C) | W (H) | W (O) |
97. Mass fraction of Nitrogen in some compound is equal to 28%. What mass of the compound contains 56 g of Nitrogen?
98. Mass fraction of Calcium in its combination with Hydrogen is 0.952. Determine the mass of Hydrogen, which is contained in 20 g of the compound.
99. Mixed 100 g of cement and 150 g of sand. What is the mass fraction of cement in the prepared mixture?
Popel P. P., Kriklya L.S., Khimia: Pidruch. for 7 cl. zagalnoosvit. navch. prl. - К .: EC "Academy", 2008. - 136 p .: il.
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