Weight of 1 mole of a substance is called its molar weight and is designated by letter M. Units of measure of molar weight – g/mol. The way of calculation of this size depends on the set conditions.
It is required to you
- - Periodic Table of D.I. Mendeleyev (Mendeleyev's table);
- - calculator.
Instruction
1. If the chemical formula of substance is known, then its molar weight can be calculated by means of Mendeleyev's table. The molar mass of substance (M) is equal to its relative molecular weight (Mr). In order that to calculate it, find atomic masses of all elements which are a part of substance (Ar) in Mendeleyev's table. Usually this number written in the bottom right corner of a cell of the corresponding element under its serial number. For example, the atomic mass of hydrogen is equal 1 - Ar (H)=1, the atomic mass of oxygen is equal 16 - Ar (O)=16, the atomic mass of sulfur is equal 32 – Ar(S)=32.
2. To learn the molecular and molar mass of substance, it is necessary to put relative atomic masses of the elements which are among it taking into account their atoms. Mr = Ar1n1+Ar2n2+ … +Arxnx. So, the molar mass of water (H2O) is equal to the sum of the atomic mass of hydrogen (H) increased by 2 and the atomic mass of oxygen (O). M (H2O) = Ar(H)? 2 + Ar(O) = 1? 2 +16=18 (g/mol). The molar mass chamoisof acids (H2SO4) is equal to the sum of the atomic mass of hydrogen (H) increased by 2, the atomic mass of sulfur (S) and atomic mass of oxygen (O) increased by 4. The M (H2SO4) =Ar (H)? 2 + Ar(S) + Ar (O)? 4=1? 2 + 32 + 16? 4 = 98 (g/mol). The molar mass of the simple substances consisting of one element is considered also. For example, the molar mass of gas of oxygen (O2) is equal to the atomic mass of an element of oxygen (O) increased by 2. M (O2) = 16? 2 = 32 (g/mol).
3. If the chemical formula of substance is unknown, but its quantity and weight is known, molar weight can be found on a formula: =m/n m, where M – molar weight, m – the mass of substance, n – amount of substance. For example, it is known that the 2nd mole of a substance has the mass of 36 g, then its molar weight is equal to M = to m/n=36 of? 2 mol = 18 g/mol (most likely it is H2O water). If 1.5 mole of a substance have the mass of 147 g, then its molar weight is equal to M = to m/n=147g? 1.5 mol = 98 g/mol (most likely it is sulfuric H2SO4 acid).