mass of the molecule, as well as the masses of atoms are very small.Therefore, to calculate them using a comparison with the atomic mass unit.Relative molecular weight compound - a physical quantity, which is the ratio of the mass of the molecule compounds to 1/12 share of carbon atoms.This figure indicates how many times the weight of the whole molecule is greater than 1/12 of the weight of an elementary particle of carbon and, like any relative value has no dimension and is denoted by Mr.
Mr (compound) = m (molecular compound) / m 1/12 (C).However, in practice, it uses a different scheme for computing this value.In accordance therewith, the relative molecular mass equal to the total value of the relative atomic masses (Ar) of all chemical elements which form the compound according to the number of elementary particles of each element, i.e.schematic can be written as:
Mr (B1xC1y) = x * Ar (B1) + y * Ar (C1).
To correctly determine this value must be:
- know the chemical formula of the substance;
- correctly determine the Ar in the Mendeleev table (so, if the number of standing after the decimal point is equal to or privyshaet 5, the rounding to the nearest whole unit added: for example, Ar (Li) = 6, 941 for the calculation using an integerwhich is equal to 7, and if the number is less than 5, so the reserve, which has: Ar (K) = 39,098, i.e. take 39).
- in calculating Mr, do not forget to take into account the number of atoms, ie,index standing at the elements in a compound.
relative molecular mass, the formula of which is schematically shown above, is used to complex compounds.Because for the calculation of this value in a simple substance sufficient to determine only the relative atomic mass of the periodic table and optionally multiplied by the number of elementary particles.For example: Mr (P) = Ar (R) = 31 and Mr (N2) = 2 * Ar (N) = 2 * 14 = 18.
Consider another example, and find out what is the relative molecular mass of water - a complicated matter.The empirical formula of the substance H2O, ie,it consists of two hydrogen atoms and one oxygen atom.Therefore recording solution looks like:
Mr (H 2 O) = 2 * Ar (H) + Ar (O) = 2 * 1 + 16 = 18
can record abbreviated, omitting the literal expression.This figure shows that Mr 18 times more than 1/12 the mass of an elementary particle of carbon.Similarly, relative molecular mass is determined by any chemical compound, provided that it is known empirical formula.But also using this value, you can restore the qualitative and quantitative composition of unknown substances to determine the content of individual nuclei.In practice, for the determination of Mr substance used physico-chemical methods such as distillation, mass spectrometry, gas chromatography, etc.To determine this index for the polymers used methods based on colligative properties (determined by the number of double bonds, functionality, viscosity, light scattering ability).
Thus, relative molecular mass characteristic of each substance and for it to be individual.This value is determined for both simple and complex compounds, inorganic and organic.Its performance is especially important in studying and synthesizing polymers, the properties of which will be dependent on molecular weight.
