The masses of molecules, like the masses of atoms, are very small. Therefore, for their calculation, a comparison with the atomic unit of mass is used. The relative molecular weight of the compound is a physical quantity that is equal to the ratio of the mass of the molecule of the compound to 1/12 of the carbon atom. This indicator indicates how many times the weight of the whole molecule exceeds 1/12 of the weight of an elementary carbon particle and, like any relative value, has no dimension and is indicated by the symbol Mr.
Mr (compounds) = m (molecules of the compound) / 1/12 m (C). However, in practice, another scheme for calculating this value is used. In accordance with it, the relative molecular mass is equal to the total value of the relative atomic masses (Ar) of all chemical elements that form this compound taking into account the number of elementary particles of each element, i.e. can be schematically written like this:
Mr (B1xC1y) = x * Ar (B1) + y * Ar (C1).
In order to correctly determine this value, it is necessary:
- know the chemical formula of a substance;
- it is correct to determine Ar in the table of D.I. equal to 7; and if the number is less than 5, then leave it as it is: Ar (K) = 39.098, i.e. take 39).
- when calculating Mr, do not forget to take into account the number of atoms, i.e. indices of the elements in the compound formula.
The relative molecular weight, the formula of which is schematically indicated above, applies to complex compounds. Because to calculate this value for a simple substance, it is enough to determine only the relative atomic mass from the periodic table and, if necessary, multiply by the number of elementary particles. For example: Mr (P) = Ar (P) = 31 and Mr (N 2 ) = 2 * Ar (N) = 2 * 14 = 18.
Consider another example and find out what the relative molecular mass of water is - a complex substance. The empirical formula of this substance is H 2 O, i.e. it consists of 2 hydrogen atoms and 1 oxygen atom. Therefore, the solution record looks like this:
Mr (H 2 O) = 2 * Ar (H) + Ar (O) = 2 * 1 + 16 = 18
You can write in abbreviation, omitting the literal expression. This figure shows that Mr is 18 times larger than 1/12 of the mass of an elementary carbon particle. The relative molecular weight of any chemical compound is determined similarly, provided that its empirical formula is known. But also, using this value, it is possible to restore the qualitative and quantitative composition of unknown substances, to establish the content of individual nuclides. In practice, physicochemical methods are used to determine Mr of a substance, such as distillation, mass spectrometry, gas chromatography , etc. To determine this indicator in polymers, methods are used based on the colligative properties of solutions (they determine the number of double bonds, functional group, viscosity, and the ability to scatter light).
Thus, the relative molecular mass is characteristic of each substance and will be individual for it. This value is determined for both simple and complex compounds, inorganic and organic. Its indicators are especially important in the study and synthesis of polymers, whose properties will depend on the molecular weight index.