Relative molecular mass is a physical quantity peculiar to each substance

The masses of the molecules, like the masses of the atoms, are very small. Therefore, for their calculation, a comparison with the atomic unit of mass is used. The relative molecular weight of a compound is a physical quantity that is equal to the ratio of the mass of the molecule of the compound to the 1/12 fraction of the carbon atom. This indicator indicates how many times the weight of the whole molecule exceeds 1/12 part of the weight of the elementary carbon particle and, like any relative value, has no dimension and is designated by the symbol "Mr.".

Mr (compound) = m (compound molecule) / 1/12 m (C). However, in practice, another scheme for calculating this quantity is used. According to it, the relative molecular mass is equal to the total value of the relative atomic masses (Ar) of all the chemical elements that form the given compound, taking into account the number of elementary particles of each element, i.e. Can be schematically written as:

Mr (B1xC1y) = x * Ar (B1) + y * Ar (C1).

In order to correctly determine this value you need:

1. Know the chemical formula of the substance;
2. Correctly determine Ar in the table of DI Mendeleyev (so if the number after the decimal equals or accustoms to 5, then when rounding to the whole is added a unit: for example, Ar (Li) = 6, 941, for calculation we use an integer that Is equal to 7, and if the number is less than 5, then leave it as it is: Ar (K) = 39,098, ie we take 39).
3. When calculating Mr, we do not forget to take into account the number of atoms, i.e. Indices that stand in the elements of the join formula.

The relative molecular weight, the formula of which is schematically indicated above, is applied to complex compounds. Because to calculate a given value for a simple substance it is sufficient to determine only the relative atomic mass by 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.

Let's consider another example and find out what the relative molecular weight 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

It can be written in abbreviated form, omitting the letter expression. This figure shows that Mr is 18 times larger than 1/12 of the mass of the elementary carbon particle. Similarly, the relative molecular weight of any chemical compound is determined, 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, such as: distillation, mass spectrometry, gas chromatography , etc., are used to determine Mr substance. To determine this index polymers use methods based on the colligative properties of solutions (determine the amount of double bonds, the functional group, viscosity, the ability to scatter light).

Thus, the relative molecular mass is inherent in 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.