In practical and theoretical chemistry, there exist and have practical significance 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 values depend on the composition of a simple or complex substance.
How to determine molar mass or molecular? Both of these physical quantities cannot (or almost impossible) be found by direct measurement, for example, by weighing a substance on a scale. 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 mass is expressed by atomic mass units, which are conventional quantities, are designated a. E. m., as well as another name - "color blind". 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 masses 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 a. eat.;
- nitrogen (N2) - 14 • 2 = 28 a. eat.;
- chlorine (Cl2) - 35 • 2 = 70 a. eat.;
- ozone (O3) - 16 • 3 = 48 a. eat.
The molecular masses of complex substances are calculated by summing the products of atomic mass by the number of atoms for each element in the molecule. 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 not difficult to do, since it is the mass per unit quantity 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 will be obtained. For example, how to find the molar mass of carbon dioxide (CO2)? It follows (12 + 16 • 2) • 1 g / mol = 44 g / mol, that is, MCO2 = 44 g / mol. For simple substances, in molecules that contain only one atom of an element, this indicator, expressed in g / moles, 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 molecular, the molar mass of a multicomponent mixture is an additive quantity. It is the sum of the products of the molar mass of the component and its share in the mixture: M = ∑Mi • Xi, that is, both the average molecular and average molar mass can be calculated.
For example, air, which includes approximately 75.5% nitrogen, 23.15% oxygen, 1.29% argon and 0.046% carbon dioxide (the remaining impurities that are contained in smaller quantities can be neglected): 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 accuracy of determining the atomic masses indicated in the periodic table is different? For some elements, it is indicated to the nearest tenth, for others to the nearest hundredth, for the third to the thousandth, and for such as radon, to the integer, for manganese to the ten thousandth.
When calculating the molar mass, it makes no sense to carry out calculations with greater accuracy than up to tenths, since they have practical applications 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 procedures are introduced with the help of certain procedures: the titer of the solution is established, calibrations are carried out using standard samples, etc.