How to find the molar mass

In practical and theoretical chemistry there are alsohave two practical concepts such as molecular (it is often replaced by the concept of molecular weight, which is not correct) and molar mass. Both these quantities depend on the composition of a simple or complex substance.

How to determine molar mass or molecular weight? Both these physical quantities can not (or almost not) be found by direct measurement, for example, by weighing a substance on a scale. They are calculated from the chemical formula of the compound and the atomic masses of all the elements. These quantities are numerically equal, but differ in dimension. Molecular mass is expressed by atomic units of mass, which are conditional, have the designation a. e. m., and also another name - "dalton". Units of molar mass are expressed in g / mol.

Molecular masses of simple substances, molecules of which consist of one atom, are equal to their atomic masses, which are indicated in the periodic table of Mendeleyev. 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 masses of simple substances,whose molecules 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 a. eat.;
nitrogen (N2) - 14 • 2 = 28 a. eat.;
chlorine (Cl2) -35 • 2 = 70 a. eat.;
Ozone (O3) - 16 • 3 = 48 a. eat.

Calculate the molecular masses of complex substances, summing up the products of the atomic mass by the number of atoms for the constituent of the molecule of each element. For example, for:

Hydrochloric acid (HCl) - 2 + 35 = 37 a. eat.;
carbon monoxide (CO) - 12 + 16 = 28 a. eat.;
carbon dioxide (CO2) - 12 + 16 • 2 = 44 a. eat.

But how to find the molar mass of substances?

This is easy to do, since it is a massunit of the amount of a specific 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 to find the molar mass of carbon dioxide (CO2)? It follows (12 + 16 • 2) • 1 g / mol = 44 g / mole, that is, MCO2 = 44 g / mol. For simple substances, in molecules that contain only one element atom, this index, expressed in grams / mole, is numerically the same as 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 with the example of oxygen: MO2 = 16 • 2 = 32 g / mol.

Here were examples for specificsimple or complex substances. But is it possible and how to find the molar mass of a product consisting of several components? Like the molecular, molar mass of a multicomponent mixture is an additive quantity. It is the sum of the products of the molar mass of the component to its fraction in the mixture: M = ΣMi · Xi, that is, the average molecular weight and the average molar mass can be calculated.

On the example of air, which includesabout 75.5% of nitrogen, 23.15% of oxygen, 1.29% of argon and 0.046% of carbon dioxide (other impurities that are contained in smaller amounts can be neglected): Air-air = 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 accuracythe determination of the atomic masses indicated in the periodic table is different? For some elements, it is indicated to within a tenth, for others to the nearest hundredth, for third to thousandth, and for such as radon to whole, for manganese to ten thousand.

When calculating the molar mass, it makes no sense to drivecalculations with greater accuracy than up to a tenth, 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 more precision is required for chemists, appropriate corrections are made by means of certain procedures: a solution titer is established, calibration is carried out according to standard samples and so on.