Relative Masses and MolesAQA GCSE Chemistry: Revision notes

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of all the atoms shown in its formula.

To calculate Mr, add up the relative atomic mass (Ar) of every atom present, remembering to multiply by the number of each atom shown in the formula.

Chemical amounts are measured in moles (mol). The mass of one mole of any substance, in grams, is numerically equal to its relative formula mass.

The number of particles in one mole of any substance is the Avogadro constant: 6.02 × 10²³.

So one mole of CO2 (Mr = 44) has a mass of 44 g and contains 6.02 × 10²³ molecules of CO2.

Use the relationship between mass, moles and Mr:

moles = mass (g) ÷ Mr

Rearranging gives:

mass (g) = moles × Mr

This lets you calculate the number of moles from a given mass, or the mass from a given number of moles, as long as you know the substance's Mr.

A balanced symbol equation shows the ratio of moles of each reactant and product. You can use this ratio, together with mole–mass conversions, to calculate the mass of any reactant or product, given the mass of one substance in the equation.

1. Convert the given mass to moles (moles = mass ÷ Mr)
2. Use the balancing numbers in the equation to find the moles of the substance you want
3. Convert that number of moles back to mass (mass = moles × Mr)

The same approach can be used in reverse — if you know the masses of reactants and products, you can convert to moles, find simple whole number ratios, and use these to balance an equation.