Calculating Masses from Balanced Equations (OCR Gateway GCSE Chemistry: Combined Science)

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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 

( 1 tonne = 1,000,000 g)

Answer

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

Examiner Tip

Don't be worried about working in mass units other than grams, as the mass ratios don't change. You can see in the second example that 51g of aluminium oxide gives 27 g of aluminium metal.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.