# Mole- Definition and molar mass calculations

Before we go into how to calculate the molar mass of a substance let’s go through a preview of what a mole is.

### Mole– Definition

This is the SI unit measure of a quantity “chemical entity”, such as atoms, electrons, or protons.

It’s standard definition defines it as the amount of substance that contains as many particles as there are atoms in 12 grams of carbon-12, so 1 mol is said to contain 6.022 × 10²³ elementary entities of the substance.

1 mole of oxygen atoms contains 6.022 × 10 ²³ oxygen atoms just as 1 dozen eggs means 12 eggs, Atoms, molecules and formula units are very small and difficult to work with, however, the mole allows a chemist to work with amounts large enough to use.

Now in a chemical reaction we can say that one mole of A will react exactly one mole of B in the reaction below to give one mole of C

A + B C

The value 6.022 × 1023 is known as Avogadro’s number, a basic measuring unit in chemistry discovered by the 18th century chemist Amadeo Avogadro. ### Calculating Molar Mass

The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol).

i.e it is the mass of one mole of that substance expressed in grams per mol (g mol −1) The molar mass of an element will be its relative atomic mas expressed in grams and the molar mass of a compound will be its relative molecular mass expressed in grams.

Example;

What is the mass of 3 moles of oxygen, (Gas O2)?

Solution;

Oxygen has an atomic mass of 16 ( O= 16) but according to the question given, the atom of oxygen exists as O2 “meaning it is diatomic”.

So since the mass of 1 mole oxygen atom is 16g, Then the mass of 1 mole Diatomic oxygen will be given as (2 × 16)g = 32g

And the mass of 3 moles of O2 will be given as (3 × 32)g = 96g

So the mass of 3 moles of oxygen O2 is 96g

Fun fact; Democritus was the one who originally introduced developed the concept of the atom. 