For the GCSE Chemistry exam, you will be tested on calculations. In chemistry, we need to know how much of each reactant to use in order to make the right amount of product. Atoms are too small to count individually, so we weigh them instead.
Calculating the number of moles of a substance allows us to get the right ratio of our reactants for our reaction to work successfully. We also use calculations to help us decide how efficient our reactions are.
Atom economy is a way of quantifying how much waste there is associated with a particular reaction. Percentage yield is all important too. Chemists spend a long time working out the best conditions in order to maximise their yield.