AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    In a balanced chemical equation, the sum of the relative formula masses of…

    Full specification wording

    Relative formula mass: In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

    A balanced equation shows the same number of each type of atom on both sides, so the total mass is conserved.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 1 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A balanced equation shows the same number of each type of atom on both sides, so the total mass is conserved. Relative formula mass, Mᵣ, is the sum of the relative atomic masses of all atoms in a formula. To check conservation, multiply each species' Mᵣ by its coefficient in the equation, then sum for reactants and for products. For example, in 2H₂ + O₂ → 2H₂O: reactants give 2 × 2 = 4 for H₂ plus 32 for O₂, totalling 36; products give 2 × 18 = 36. The totals match, confirming mass conservation. This method also supports reacting mass calculations and shows why coefficients matter in quantitative work.

    What earns the marks

    • Relative formula mass is the sum of relative atomic masses of all atoms in the formula.
    • Each species' Mᵣ must be multiplied by its coefficient in the balanced equation.
    • The total Mᵣ of reactants equals the total Mᵣ of products when the equation is balanced.
    • Conservation of mass follows from equal numbers of each atom on both sides.
    • Worked check: 2H₂ + O₂ → 2H₂O gives 36 = 36 using Mᵣ values H₂ = 2, O₂ = 32, H₂O = 18.

    Common mistakes and how to put them right

    • Forgetting to multiply Mᵣ by the coefficient, so comparing 2 + 32 with 18.

      Put it right: Include coefficients, giving 36 = 36.

    • Adding relative atomic masses incorrectly, such as using the wrong value for an element.

      Put it right: Use the periodic table values accurately and sum every atom.

    • Assuming an unbalanced equation still conserves mass.

      Put it right: Coefficients must first balance the equation before totals can be compared.

    Exam tips

    • Write the Mᵣ of each species clearly beside its formula before multiplying by coefficients.
    • Show the multiplication step, for example 2 × 18 = 36, so the examiner can follow your reasoning.
    • Check that the equation is balanced before using it for mass calculations.
    • Use a periodic table with relative atomic masses and round consistently as instructed.

    Try the lesson

    The start of the Relative formula mass: In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown. lesson, made from checked teaching notes for this specification point.

    Have a go first

    For the balanced equation 2Mg + O₂ → 2MgO, the total relative formula mass of the reactants is 80. What is the total relative formula mass of the products?

    Listen

    A balanced equation shows the same number of each type of atom on both sides, so the total mass is conserved. Relative formula mass, Mᵣ, is the sum of the relative atomic masses of all atoms in a formula. To check conservation, multiply each species' Mᵣ by its coefficient in the equation, then sum for reactants and for products. For example, in 2H₂ + O₂ → 2H₂O: reactants give 2 × 2 = 4 for H₂ plus 32 for O₂, totalling 36; products give 2 × 18 = 36. The totals match, confirming mass conservation. This method also supports reacting mass calculations and shows why coefficients matter in quantitative work. A strong answer earns marks for these points. Relative formula mass is the sum of relative atomic masses of all atoms in the formula. Each species' Mᵣ must be multiplied by its coefficient in the balanced equation. The total Mᵣ of reactants equals the total Mᵣ of products when the equation is balanced. Conservation of mass follows from equal numbers of each atom on both sides. A common slip: Forgetting to multiply Mᵣ by the coefficient, so comparing 2 + 32 with 18. Instead, include coefficients, giving 36 = 36. Exam tip: Write the Mᵣ of each species clearly beside its formula before multiplying by coefficients.

    See it

    A top-down flowchart. The first box states the balanced equation 2H₂ + O₂ → 2H₂O. An arrow leads to a box saying to multiply each Mᵣ by its coefficient. From this, three arrows lead to three boxes: reactants 2 × 2 = 4 for H₂, reactants 32 for O₂, and products 2 × 18 = 36 for H₂O. The two reactant boxes both lead to a box reading total reactants = 36, and the product box leads to a box reading total products = 36. Both of these final boxes lead to a last box reading totals match: mass conserved.
    It shows how multiplying each species' relative formula mass by its coefficient gives equal totals of 36 for reactants and products, confirming conservation of mass.

    Key words

    Say what each word means before you turn the card.

    Relative formula mass

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    Conservation of mass

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    Balanced equations and coefficients

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    Quantitative reasoning

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    See every definition
    Relative formula mass
    The sum of the relative atomic masses of all atoms in a formula, given as Mᵣ.
    Conservation of mass
    A balanced equation shows the same number of each type of atom on both sides, so the total mass is conserved.
    Balanced equations and coefficients
    Each species' Mᵣ must be multiplied by its coefficient in the balanced equation, and the equation must be balanced before totals can be compared.
    Quantitative reasoning
    Multiplying each species' Mᵣ by its coefficient and summing for reactants and products supports reacting mass calculations and shows why coefficients matter in quantitative work.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box states the balanced equation 2H₂ + O₂ → 2H₂O. An arrow leads to a box saying to multiply each Mᵣ by its coefficient. From this, three arrows lead to three boxes: reactants 2 × 2 = 4 for H₂, reactants 32 for O₂, and products 2 × 18 = 36 for H₂O. The two reactant boxes both lead to a box reading total reactants = 36, and the product box leads to a box reading total products = 36. Both of these final boxes lead to a last box reading totals match: mass conserved.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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