Reacting Masses

This lesson covers reacting mass calculations as required by the AQA GCSE Chemistry specification (4.3.2). You need to be able to use balanced equations and relative formula masses to calculate the masses of reactants and products in chemical reactions. These calculations are essential for predicting how much product you can make from a given amount of reactant, or how much reactant you need.

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The Principle of Reacting Masses

In a balanced chemical equation, the coefficients (large numbers in front of the formulae) tell you the ratio of moles in which the substances react and are produced. Because we know the relationship between moles and mass (moles = mass / Mr), we can convert between moles and grams to find unknown masses.

Step	What to Do
1	Write the balanced symbol equation
2	Identify the substance you know and the substance you want to find
3	Calculate the Mr of both substances
4	Use the balanced equation to find the mole ratio
5	Calculate the unknown mass