IGCSEChemistryOxford AQARevision NotesQuantitative ChemistryConservation of Mass Including the Quantitative Interpretation of Chemical EquationsReacting Masses

Reacting Masses (Oxford AQA IGCSE Chemistry): Revision Note

Exam code: 9202

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Updated on

Oxford AQA ChemistryOxford AQA Combined Science Double Award: Chemistry

Reacting Masses

  • Chemical / symbol equations can be used to calculate:

    • The moles of reactants and products 

    • The mass of reactants and products

  • To do this:

    • Information from 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

Magnesium undergoes combustion to produce magnesium oxide.

The overall reaction that is taking place is shown in the equation below.

2Mg (s) + O2 (g)  ⟶ 2 MgO (s) 

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

Ar(O) = 16   Ar(Mg) = 24

Answer:

  • Step 1 - calculate the moles of magnesium 

    • Moles = begin mathsize 14px style open parentheses mass over M subscript r close parentheses end style = open parentheses 6 over 24 close parentheses = 0.25

  • Step 2 - use the molar ratio from the balanced symbol equation

    • 2 moles of magnesium produce 2 moles of magnesium oxide

    • The ratio is 1 : 1

    • Therefore, 0.25 moles of magnesium oxide is produced

  • Step 3 - calculate the mass of magnesium oxide

    • Mass = moles x Mr = 0.25 moles x (24 + 16) = 10 g

Worked Example

In theory, aluminium could decompose as shown in the equation below.

2Al2O3  ⟶  4Al +  3O2 

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

Ar(O) = 16   Ar(Al) = 27

Answer:

  • Step 1 - calculate the moles of aluminium oxide 

    • Mass = 51 tonnes x 106 = 51 000 000 g

    • Moles = open parentheses mass over M subscript r close parentheses = open parentheses 51000000 over 102 close parentheses = 500 000

  • Step 2 - use the molar ratio from the balanced symbol equation

    • 2 moles of aluminium oxide produces 4 moles of aluminium 

    • The ratio is 1 : 2

    • Therefore, 2 x 500 000 = 1 000 000 moles of aluminium is produced

  • Step 3 - calculate the mass of aluminium 

    • Mass = moles x Mr = 1 000 000 moles x 27 = 27 000 000 g

    • Mass in tonnes = open parentheses 27000000 over 10 to the power of 6 close parentheses = 27 tonnes

Reaction Yield

  • Yield is the term used to describe the amount of product you get from a reaction

  • Even though no atoms are lost or gained in a chemical reaction, it is not always possible to obtain the calculated amount of product

  • This is because:

    • Some reactants may be left behind in the equipment

    • The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants

    • Some products may also be lost during separation and purification stages such as filtration or distillation

    • Some of the reactants may react in ways different from the expected reaction such as a substance reacting with an impurity in one of the reactants

Examiner Tips and Tricks

Make sure you can give at least two reasons why you don't always obtain the expected amount of product for a simple recall style question.

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    Author
    Reviewer
    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry Content Creator / Senior Marketing Executive

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

    Stewart Hird

    Reviewer: Stewart Hird

    Expertise: Chemistry Content Creator

    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.