AQA GCSE Chemistry

Revision Notes

3.2.2 Amounts of Substances in Equations

Higher Tier Only

Calculating Masses from Balanced Equations

  • Chemical equations can be used to calculate the moles or masses of reactants and products
  • To do this, information given in the question is used to find the amount in moles of the substances being considered
  • Then, the ratio between the substances is identified using the balanced chemical equation
  • Once the moles have been determined they can then be converted into grams using the relative atomic or relative formula masses

Worked Example

Example 1

Calculate the mass of magnesium oxide that can be made by completely burning 6.0 g of magnesium in oxygen in the following reaction:

2Mg (s) + O2 (g)  ⟶ 2 MgO (s) 

Answer

Reacting Masses WE 1, downloadable IGCSE & GCSE Chemistry revision notes

Worked Example

Example 2

Calculate the mass of aluminium, in tonnes, that can be produced from 51 tonnes of aluminium oxide. The equation for the reaction is:

2Al2O3  ⟶  4Al +  3O2 

Answer

Reacting Masses WE 2 1, downloadable IGCSE & GCSE Chemistry revision notesReacting Masses WE 2 2, downloadable IGCSE & GCSE Chemistry revision notes

Balancing Equations using Reacting Masses

  • If the masses of reactants and products of a reaction are known then we can use them to write a balanced equation for that reaction
  • This is done by converting the masses to moles and simplifying to find the molar ratios

Worked Example

Example 3

A student reacts 1.2 g of carbon with 16.2 g of zinc oxide. The resulting products are 4.4 g of carbon dioxide and 13 g of zinc. Determine the balanced equation for the reaction.

Answer

Balancing Equations from Reacting Masses WE 1, downloadable IGCSE & GCSE Chemistry revision notesBalancing Equations from Reacting Masses WE 2, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

These questions look hard but they are actually quite easy to do, as long as you follow the steps and organise your work neatly.
Remember the molar ratio of a balanced equation gives you the ratio of the amounts of each substance in the reaction.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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