Anonymous
To do this, you need to use the periodic table. In the cell of any element, a number is given, most often, with an accuracy of 3-4 decimal places - this is the relative molecular mass (molar mass) of this element. Usually, the molecular weight is rounded up according to the appropriate mathematical rules, with the exception of chlorine - the molecular weight of a chlorine atom is 35.5. The molecular weight of a complex substance is equal to the sum of the molecular weights of its constituent elements. For example, water is H2O. The molecular weight of hydrogen is 1, oxygen - 16. This means that the molecular weight of water is 2 * 1 + 16 = 18 g / mol.
Anonymous
To determine the molar mass of substances, you must:
- have a table of the periodic table of chemical elements D.I. Mendeleev;
- know the number of atoms of each element, in the formula of the substance in question;
- know the definition of the concepts "molar mass", "mol".
Formula of substance
To describe a substance, it is necessary to know how many atoms and what type one molecule of the substance under consideration contains. For example, the inert gas krypton exists under normal conditions (atmospheric pressure 101325 Pa = 760 mm Hg, temperature 273.15 K = 0 ° C) in the atomic form of Kr. The carbon monoxide molecule consists of two carbon atoms C and an oxygen atom O: CO2. And the coolant of the refrigerator - freon 134 - has a more complex formula: СF3CFH2.
Definitions
Molar mass Mr is the mass of one mole of a substance, measured in g / mol.
Mole is the amount of a substance that contains a certain number of atoms of a given type. It is defined as the number of atoms in 12 g of the carbon isotope C-12 and is equal to Avogadro's constant N = 6.022 * 10 ^ 23 1 / mol.
Calculating molar mass
To determine the molar mass Mr of a substance, it is necessary to find out the atomic mass Ar of each element included in the substance, using the table of the periodic system of chemical elements of D.I. Mendeleev, and know the number of atoms of each element.
For example, the molar mass Mr of sodium tetraborate Na2B4O7 * 10 H2O is:
M r (Na2B4O7 * 10 H2O) = 2 * Ar (Na) + 4 * Ar (B) + 7 * Ar (O) + 10 * 2 * Ar (H) + 10 * Ar (O) = 2 * 23 + 4 * 11 + 7 * 16 + 10 * 2 * 1 * 16 = 223 g / mol.
23 Aug 2012In practical and theoretical chemistry, there are two such concepts as molecular (it is often replaced by the concept of molecular weight, which is not correct) and molar mass. Both of these quantities depend on the composition of a simple or complex substance.
How to determine molar mass or molecular mass? Both of these physical quantities cannot (or almost cannot) be found by direct measurement, for example, by weighing a substance on a balance. They are calculated based on the chemical formula of the compound and the atomic masses of all elements. These values are numerically equal, but differ in dimension. Molecular weight is expressed in atomic mass units, which are a conventional value, have the designation a. e. m., and also another name - "dalton". Molar mass units are expressed in g / mol.
The molecular masses of simple substances, the molecules of which consist of one atom, are equal to their atomic masses, which are indicated in the periodic table. For example, for:
- sodium (Na) - 22.99 a. eat.;
- iron (Fe) - 55.85 a. eat.;
- sulfur (S) - 32.064 a. eat.;
- argon (Ar) - 39.948 a. eat.;
- potassium (K) - 39.102 a. eat.
Also, the molecular weights of simple substances, the molecules of which consist of several atoms of a chemical element, are calculated as the product of the atomic mass of the element by the number of atoms in the molecule. For example, for:
- oxygen (O2) - 16. 2 = 32 amu. eat.;
- nitrogen (N2) - 14 .2 = 28 amu. eat.;
- chlorine (Cl2) - 35. 2 = 70 amu. eat.;
- ozone (O3) - 16. 3 = 48 amu. eat.
The molecular weights of complex substances are calculated by summing the products of the atomic mass and the number of atoms for each element included in the molecule. For example, for:
- hydrochloric acid (HCl) - 2 + 35 = 37 a. eat.;
- carbon monoxide (CO) - 12 + 16 = 28 amu. eat.;
- carbon dioxide (CO2) - 12 + 16. 2 = 44 amu. eat.
But how to find the molar mass of substances?
This is not difficult to do, since it is the mass of a unit of the amount of a particular substance, expressed in moles. That is, if the calculated molecular weight of each substance is multiplied by a constant value equal to 1 g / mol, then its molar mass is obtained. For example, how do you find the molar mass of carbon dioxide (CO2)? It follows (12 + 16. 2) .1 g / mol = 44 g / mol, that is, MSO2 = 44 g / mol. For simple substances, molecules containing only one atom of an element, this indicator, expressed in g / mol, numerically coincides with the atomic mass of the element. For example, for sulfur, MS = 32.064 g / mol. How to find the molar mass of a simple substance, the molecule of which consists of several atoms, can be considered using oxygen as an example: MO2 = 16. 2 = 32 g / mol.
Examples have been given here for specific simple or complex substances. But is it possible and how to find the molar mass of a product consisting of several components? Like the molecular weight, the molar mass of a multicomponent mixture is additive. It is the sum of the products of the molar mass of a component by its fraction in the mixture: M = ∑Mi. Xi, that is, both the average molecular weight and the average molar mass can be calculated.
For example, air, which contains about 75.5% nitrogen, 23.15% oxygen, 1.29% argon and 0.046% carbon dioxide (the rest of the impurities, which are contained in smaller quantities, can be neglected): Mair = 28. 0.755 + 32. 0.2315 + 40. 0.129 + 44. 0.00046 = 29.08424 g / mol ≈ 29 g / mol.
How to find the molar mass of a substance if the accuracy of determining the atomic masses indicated in the periodic table is different? For some elements, it is indicated with an accuracy of tenths, for others, with an accuracy of hundredths, for still others, up to thousandths, and for such as radon - up to whole, for manganese, up to ten thousandths.
When calculating the molar mass, it makes no sense to carry out calculations with greater accuracy than up to tenths, since they have practical application when the purity of the chemicals or reagents themselves will introduce a large error. All these calculations are approximate. But where chemists require greater accuracy, appropriate corrections are made using certain procedures: the titer of the solution is established, calibrations are performed using standard samples, and so on.
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Any substance consists of particles of a certain structure (molecules or atoms). The molar mass of a simple compound is calculated using the periodic table of elements of D.I. Mendeleev. If it is necessary to find out this parameter in a complex substance, then the calculation is long, and in this case the figure is looked at in a reference book or a chemical catalog, in particular Sigma-Aldrich.
Molar mass concept
Molar mass (M) is the weight of one mole of a substance. This parameter for each atom can be found in the periodic table of elements, it is located right below the name. When calculating the mass of compounds, the figure is usually rounded to the nearest whole or tenth. For a final understanding of where this value comes from, it is necessary to understand the concept of "mole". This is the amount of substance containing the number of particles of the latter, equal to 12 g of the stable isotope of carbon (12 C). Atoms and molecules of substances vary in size over a wide range, while their number in a mole is constant, but the mass increases and, accordingly, the volume.
The concept of "molar mass" is closely related to the Avogadro number (6.02 x 10 23 mol -1). This figure denotes a constant number of units (atoms, molecules) of a substance in 1 mole.
The value of molar mass for chemistry
Chemicals enter into various reactions with each other. Usually, the equation for any chemical interaction indicates how many molecules or atoms are used. Such designations are called stoichiometric coefficients. They usually appear before the formula. Therefore, the quantitative characterization of reactions is based on the amount of substance and molar mass. They clearly reflect the interaction of atoms and molecules with each other.
Calculating molar mass
The atomic composition of any substance or mixture of components of a known structure can be viewed from the periodic table of elements. Inorganic compounds, as a rule, are written by the gross formula, that is, without designating the structure, but only the number of atoms in the molecule. Organic substances for calculating molar mass are indicated in the same way. For example, benzene (C 6 H 6).
How is molar mass calculated? The formula includes the type and number of atoms in a molecule. According to the table by D.I. Mendeleev, the molar masses of the elements are checked, and each digit is multiplied by the number of atoms in the formula.
Based on the molecular weight and type of atoms, you can calculate their number in a molecule and draw up a compound formula.
Molar mass of elements
Often, to carry out reactions, calculations in analytical chemistry, and the placement of coefficients in equations, knowledge of the molecular mass of elements is required. If the molecule contains one atom, then this value will be equal to that of the substance. In the presence of two or more elements, the molar mass is multiplied by their number.
The value of molar mass when calculating concentrations
This parameter is used to recalculate almost all methods of expressing the concentration of substances. For example, situations often arise when determining the mass fraction based on the amount of a substance in a solution. The last parameter is expressed in the unit mol / liter. To determine the desired weight, the amount of the substance is multiplied by the molar mass. The value obtained is reduced by a factor of 10.
Molar mass is used to calculate the normality of a substance. This parameter is used in analytical chemistry for performing titri- and gravimetric analysis methods when an accurate reaction is required.
Measurement of molar mass
The first historical experience was to measure the density of gases in relation to hydrogen. Further studies of colligative properties were carried out. These include, for example, osmotic pressure, determination of the boiling or freezing difference between a solution and a pure solvent. These parameters directly correlate with the number of particles of matter in the system.
Sometimes the measurement of molar mass is carried out for a substance of unknown composition. Previously, a method such as isothermal distillation was used. Its essence lies in placing a solution of a substance in a chamber saturated with solvent vapors. Under these conditions, vapor condensation occurs and the temperature of the mixture rises, reaches equilibrium and begins to decrease. The released heat of vaporization is calculated from the change in the heating and cooling rate of the solution.
The main modern method for measuring molar mass is mass spectrometry. This is the main way to identify mixtures of substances. With the help of modern devices, this process occurs automatically, only initially it is necessary to select the conditions for the separation of compounds in the sample. The mass spectrometry method is based on the ionization of a substance. As a result, various charged fragments of the compound are formed. The mass spectrum indicates the ratio of the mass to the charge of the ions.
Determination of molar mass for gases
The molar mass of any gas or vapor is easy to measure. It is enough to use control. One and the same volume of a gaseous substance is equal in quantity to another substance at the same temperature. A known method for measuring the volume of steam is to determine the amount of displaced air. This process is carried out using a lateral arm leading to the measuring device.
Practical use of molar mass
Thus, the concept of molar mass is widely used in chemistry. To describe the process, create polymer complexes and other reactions, it is necessary to calculate this parameter. An important point is to determine the concentration of an active substance in a pharmaceutical substance. For example, using a cell culture, the physiological properties of a new compound are investigated. In addition, molar mass is important in biochemical research. For example, when studying the participation in the metabolic processes of an element. Now the structure of many enzymes is known, so it is possible to calculate their molecular weight, which is mainly measured in kilodaltons (kDa). Today, the molecular weights of almost all components of human blood are known, in particular, hemoglobin. Molecular and molar mass of a substance in certain cases are synonymous. Their differences lie in the fact that the last parameter is the average for all isotopes of the atom.
Any microbiological experiments in the precise determination of the effect of a substance on the enzyme system are carried out using molar concentrations. For example, in biocatalysis and other fields where enzymatic activity research is needed, concepts such as inducers and inhibitors are used. To regulate the activity of the enzyme at the biochemical level, it is necessary to study using precisely molar masses. This parameter has become firmly established in the field of natural and engineering sciences such as physics, chemistry, biochemistry, biotechnology. The processes, characterized in this way, become more understandable from the point of view of mechanisms, determination of their parameters. The transition from fundamental science to applied science is not complete without an indicator of molar mass, starting from physiological solutions, buffer systems and ending with the determination of dosages of pharmaceutical substances for the body.
One of the basic units in the International System of Units (SI) is the unit of the amount of substance is the mole.
Moth – this is the amount of a substance that contains as many structural units of a given substance (molecules, atoms, ions, etc.) as there are carbon atoms in 0.012 kg (12 g) of the carbon isotope 12 WITH .
Considering that the value of the absolute atomic mass for carbon is m(C) = 1.99 10 26 kg, you can calculate the number of carbon atoms N A contained in 0.012 kg of carbon.
A mole of any substance contains the same number of particles of this substance (structural units). The number of structural units contained in a substance in the amount of one mole is 6.02 10 23 and called Avogadro's number (N A ).
For example, one mole of copper contains 6.02 · 10 23 copper atoms (Cu), and one mole of hydrogen (H 2) contains 6.02 · 10 23 hydrogen molecules.
Molar mass(M) is the mass of a substance taken in an amount of 1 mol.
The molar mass is designated by the letter M and has the dimension [g / mol]. In physics, the dimension [kg / kmol] is used.
In the general case, the numerical value of the molar mass of a substance numerically coincides with the value of its relative molecular (relative atomic) mass.
For example, the relative molecular weight of water is:
Мr (Н 2 О) = 2Аr (Н) + Аr (O) = 2 ∙ 1 + 16 = 18 amu
The molar mass of water has the same value, but is expressed in g / mol:
M (H 2 O) = 18 g / mol.
Thus, a mole of water containing 6.02 · 10 23 water molecules (respectively 2 · 6.02 · 10 23 hydrogen atoms and 6.02 · 10 23 oxygen atoms) has a mass of 18 grams. In water, the amount of substance is 1 mol, contains 2 mol of hydrogen atoms and one mol of oxygen atoms.
1.3.4. The relationship between the mass of a substance and its amount
Knowing the mass of a substance and its chemical formula, and hence the value of its molar mass, it is possible to determine the amount of a substance and, conversely, knowing the amount of a substance, it is possible to determine its mass. For such calculations, you should use the formulas:
where ν is the amount of substance, [mol]; m- mass of substance, [g] or [kg]; M is the molar mass of the substance, [g / mol] or [kg / kmol].
For example, to find the mass of sodium sulfate (Na 2 SO 4) in the amount of 5 mol, we find:
1) the value of the relative molecular weight of Na 2 SO 4, which is the sum of the rounded values of the relative atomic masses:
Мr (Na 2 SO 4) = 2Аr (Na) + Аr (S) + 4Аr (O) = 142,
2) the numerically equal value of the molar mass of the substance:
M (Na 2 SO 4) = 142 g / mol,
3) and, finally, the mass of 5 mol of sodium sulfate:
m = ν M = 5 mol 142 g / mol = 710 g.
Answer: 710.
1.3.5. The relationship between the volume of a substance and its amount
Under normal conditions (n.o.), i.e. at pressure R equal to 101325 Pa (760 mm Hg), and a temperature T, equal to 273.15 K (0 С), one mole of different gases and vapors occupies the same volume, equal to 22.4 l.
The volume occupied by 1 mole of gas or vapor at normal conditions is called molar volumegas and has a dimension of liter per mole.
V mol = 22.4 l / mol.
Knowing the amount of gaseous substance (ν ) and molar volume value (V mol) you can calculate its volume (V) under normal conditions:
V = ν V mol,
where ν is the amount of substance [mol]; V is the volume of the gaseous substance [l]; V mol = 22.4 l / mol.
And, conversely, knowing the volume ( V) of a gaseous substance under normal conditions, you can calculate its amount (ν) :
Molar mass is a quantity that is most often found in calculations in chemical practice. It denotes the mass of an amount of a substance.
- The formula of a molecule of a substance, where the number of atoms of the elements included in the molecule is indicated.
- The atomic weight of each of the elements that make up the molecule. It can be found in the periodic table of the elements of Mendeleev, who has already calculated it for us.
We summarize the results obtained to obtain the total mass of the molecule.
The resulting mass of the molecule is multiplied by 1 g / mol to obtain the molar mass. 1 g / mol is a molar mass constant and represents the mass of one mole of a substance.
Since the algorithm for calculating the molar mass is very simple, if you have the Internet at hand, you can use an online calculator for simple formulas.
For more serious formulas, such as CO2H.N: N.CO2H, in order to increase reliability, it is worth doing the calculation yourself, not trusting the calculator developers.
