The masses of molecules, like the masses of atoms, are very small. Therefore, for their calculation, a comparison with an atomic mass unit is used. The relative molecular weight of a compound is a physical quantity that is equal to the ratio of the mass of a compound molecule to 1/12 of a carbon atom. This indicator indicates how many times the weight of the entire molecule exceeds 1/12 of the weight of an elementary particle of carbon and, like any relative value, has no dimension and is denoted by the symbol Mr.
Mr(compounds)=m(compound molecules) / 1/12 m(C). However, in practice, a different scheme for calculating this value is used. In accordance with it, the relative molecular weight is equal to the total value of the relative atomic masses (Ar) of all chemical elements that form a given compound, taking into account the number of elementary particles of each element, i.e. schematically can be written like this:
Mr(B1xC1y)=xAr(B1) +yAr(C1).
In order to correctly determine this value, you must:
- know the chemical formula of a substance;
- correctly determine Ar in the table of D. I. Mendeleev (so, if the number after the decimal point is equal to or greater than 5, then one is added when rounding to a whole number: for example, Ar (Li)=6, 941, for calculation use an integer that is 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. indexes of the elements in the join formula.
Relative molecular weight, the formula of which is shown schematically above, applies to complex compounds. Because to calculate this value for a simple substance, it is enough to determine only the relative atomic mass according to the periodic table and, if necessary, multiply by the number of elementary particles. For example: Mr(P)=Ar (P)=31 and Mr(N2)=2 Ar (N)=214=18.
Let's consider another example and find out what is the relative molecular weight of water - a complex substance. The empirical formula of this substance is H2O, i.e. it consists of 2 hydrogen atoms and 1 oxygen atom. Therefore, the solution entry looks like this:
Mr (H2O)=2Ar(H)+ Ar(O)=21+16=18
Can be abbreviated, omitting the literal expression. This figure shows that Mr is 18 times greater than 1/12 of the mass of an elementary particle of carbon. 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, physical and chemical methods are used to determine Mr of a substance, such as distillation, mass spectrometry, gas chromatography, etc. To determine this indicator for polymers, methods are used based on the colligative properties of solutions (they determine the number of double bonds, functional group, viscosity, ability to scatter light).
Thus, the relative molecular weight 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 performance is especially important in the study and synthesis of polymers, the properties of which will depend on the molecular weight index.
