No atoms are created or destroyed during a chemical reaction. This allows calculations to be made on the quantities of substances that react and of the products made.
There is a relationship between a relative atomic mass (Ar) or relative formula mass (Mr), and the mass of one mole of a substance:
Some examples are given in the table below.
| Substance | Formula | Ar or Mr | Mass of 1 mol (g) |
|---|---|---|---|
| Calcium | Ca | 40.1 | 40.1 |
| Oxygen | O2 | 2 × 16.0 = 32.0 | 32.0 |
| Calcium carbonate | CaCO3 | 40.1 + 12.0 + (3 × 16.0) = 100.1 | 100.1 |
This relationship is useful because it makes it possible to work out the mass of reactants needed to make a particular mass of product. It is also possible to work out which reactant will be used up.
The number of moles of a given mass substance is calculated using:
Use Ar instead of Mr for metals or separate atoms.
Calculate the number of moles of carbon atoms in 6.0 g of carbon. (Relative atomic mass: C = 12.0)
number of moles = \(\frac{mass}{A_r}\)
number of moles = \(\frac{6.0}{12.0}\)
= 0.50 mol
To calculate the mass if given the number of moles and the relative formula mass, the equation must be rearranged to make mass the subject.
One way to remember this is ‘mass equals mister mole’.
Calculate the mass of 0.25 mol of carbon dioxide molecules. (Relative formula mass: CO2 = 44.0)
mass = 44.0 × 0.25
= 11 g
The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used instead of an Mr.
Calculate the mass of 0.125 moles of iron. (Relative atomic mass: Fe = 55.8)
mass = 55.8 × 0.125
= 6.98 g (to 3 significant figures)