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GCSEChemistryEdexcelRevision Notes1. Atomic StructureCalculations Involving MassesReacting Masses

Reacting Masses (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

Last updated

Reacting Masses

  • Chemical equations can be used to calculate the reacting masses of reactants and products

  • The mass ratio between the substances is identified using the balanced chemical equation

  • The steps are:

    1. Write down the balanced equation for the reaction

    2. Write the relative formula masses under the substances and add the units in the question

    3. Multiply the relative formula masses by the coefficients in the equation

    4. Find the mass of product for 1 g of reactant

    5. Scale up for the mass given in the question

Worked Example

Example 1

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

Answer:

  • Step 1: Write the balanced equation

2Mg (s) + O2 (g)  ⟶ 2MgO (s)

  • Step 2: Add RFMs and units

 24 g

40 g

  • Step 3: Multiply by coefficients

 2 x 24 = 48 g  

2 x 40 = 80 g

  • Step 4: Cross multiply for 1 g

1 g

80 / 48 = 1.66 g

  • Step 5: Scale up to mass in question

6 g

6 x 1.66 = 10 g

                                           

Worked Example

Example 2

Calculate the mass of aluminium, in tonnes, that can be produced from 51 tonnes of aluminium oxide.

Answer:

  • Step 1: Write the balanced equation

2Al2O3       ⟶     4Al       +       3O2

  • Step 2: Add the RFMs and units

 102 tonnes

27 tonnes

  • Step 3: Multiply by coefficients

2 x 102 = 204 tonnes

4 x 27 = 108 tonnes

  • Step 4: Cross multiply for 1 tonne

 1 tonne

108 / 204 = 0.53 tonne

  • Step 5: Scale up to mass in question

51 tonnes

51 x 0.53 = 27 tonnes of Al

Examiner Tips and Tricks

As long as you are consistent it doesn't matter whether you work in grams or tonnes or any other mass unit as the reacting masses will always be in proportion to the balanced equation.

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Percentage Composition

  • The percentage by mass of an element in a compound can be calculated using the following equation:

begin mathsize 14px style percent sign space mass space of space an space element equals fraction numerator A subscript straight r cross times number space of space atoms space of space the space element over denominator M subscript straight r space of space the space compound end fraction cross times 100 end style

Worked Example

Calculate the percentage by mass of calcium in calcium carbonate, CaCO3.

Answer:

Calculate % Mass of an Element in a Compound WE, downloadable IGCSE & GCSE Chemistry revision notes

Examiner Tips and Tricks

Don’t forget to multiply your answer by 100 in order to convert it to a percentage.

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    Stewart Hird

    Author: Stewart Hird

    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 Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.