AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The term relative formula mass will be used for ionic compounds.

    Full specification wording

    Relative atomic mass and relative molecular mass: The term relative formula mass will be used for ionic compounds.

    Ionic compounds are giant lattices, not collections of molecules, so there is no molecule whose mass could be quoted.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Ionic compounds are giant lattices, not collections of molecules, so there is no molecule whose mass could be quoted. The term relative formula mass is used instead: it is the sum of the relative atomic masses of the ions in the simplest formula unit. Sodium chloride is 23.0⁺ 35.5 = 58.5, and calcium chloride is 40.1⁺ 2 × 35.5 = 111.1. Where a formula contains a bracketed compound ion, everything inside the bracket is multiplied by the subscript outside, so calcium hydroxide is 40.1⁺ 2(16.0⁺ 1.0) = 74.1. Hydrated salts include the water: the dot in the formula means add the mass of that many water molecules. Like relative molecular mass, the value is a ratio with no units.

    What earns the marks

    • Adding the relative atomic masses of all the ions in the formula unit.
    • Multiplying everything inside a bracket by the subscript outside it.
    • Including the water of crystallisation shown after the dot in a hydrated formula.
    • The value quoted without units.

    Common mistakes and how to put them right

    • Multiplying only the first atom inside a bracket, so calcium hydroxide comes out too light.

    • Treating sodium chloride as a molecule and calling the answer a relative molecular mass.

    • Leaving out the water of crystallisation in a hydrated salt.

    • Adding the subscripts instead of using them as multipliers.

    Exam tips

    • Expand any bracketed formula on the page before you add anything up.
    • Use the relative atomic masses printed in the data booklet, not rounded whole numbers, when the answer needs decimals.
    • Say relative formula mass rather than relative molecular mass whenever the substance is ionic.

    Try the lesson

    The start of the Relative atomic mass and relative molecular mass: The term relative formula mass will be used for ionic compounds. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which term is used for the mass of one formula unit of an ionic compound such as sodium chloride?

    Listen

    Ionic compounds are giant lattices, not collections of molecules, so there is no molecule whose mass could be quoted. The term relative formula mass is used instead: it is the sum of the relative atomic masses of the ions in the simplest formula unit. Sodium chloride is 23.0⁺ 35.5 = 58.5, and calcium chloride is 40.1⁺ 2 × 35.5 = 111.1. Where a formula contains a bracketed compound ion, everything inside the bracket is multiplied by the subscript outside, so calcium hydroxide is 40.1⁺ 2(16.0⁺ 1.0) = 74.1. Hydrated salts include the water: the dot in the formula means add the mass of that many water molecules. Like relative molecular mass, the value is a ratio with no units. A strong answer earns marks for these points. adding the relative atomic masses of all the ions in the formula unit. multiplying everything inside a bracket by the subscript outside it. including the water of crystallisation shown after the dot in a hydrated formula. the value quoted without units. Exam tip: Expand any bracketed formula on the page before you add anything up.

    See it

    A top-down flowchart. The first box states that an ionic compound is a giant lattice with no molecules. An arrow leads to a box saying to use relative formula mass. An arrow leads to a box saying to add the relative atomic masses of all the ions in the formula unit. From that box, three arrows lead to three separate boxes: one says that for a bracket, everything inside is multiplied by the subscript outside; one says that for a hydrated salt, the dot means adding the mass of that many water molecules; and one says the value is a ratio with no units.
    It shows why relative formula mass is used for ionic compounds and the steps for calculating it, including brackets, water of crystallisation and no units.

    Key words

    Say what each word means before you turn the card.

    relative formula mass

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    ionic lattices

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    formula units

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    water of crystallisation

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    See every definition
    relative formula mass
    The sum of the relative atomic masses of the ions in the simplest formula unit, used for ionic compounds.
    ionic lattices
    Giant lattices that ionic compounds form, rather than collections of molecules, so there is no molecule whose mass could be quoted.
    formula units
    The simplest unit of an ionic compound, whose ions' relative atomic masses are added to give the relative formula mass.
    water of crystallisation
    The water shown after the dot in a hydrated formula, whose mass must be included in the calculation.

    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 that an ionic compound is a giant lattice with no molecules. An arrow leads to a box saying to use relative formula mass. An arrow leads to a box saying to add the relative atomic masses of all the ions in the formula unit. From that box, three arrows lead to three separate boxes: one says that for a bracket, everything inside is multiplied by the subscript outside; one says that for a hydrated salt, the dot means adding the mass of that many water molecules; and one says the value is a ratio with no units.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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