AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: use the equation in calculations.

    Full specification wording

    The ideal gas equation: Students should be able to: use the equation in calculations.

    A typical calculation chains the gas equation to a mole calculation.

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

    • Point 2 of 2
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A typical calculation chains the gas equation to a mole calculation. Given a mass of propanone, first find the amount using n = mass ÷ molar mass. Then convert the data: 95 °C becomes 368 K and 103 kPa becomes 103000 Pa. Rearranging gives V = nRT/p, which returns a volume in cubic metres; multiplying by 10⁶ converts to the cubic centimetres the question wanted, about 75 cm³. Working the other way, a measured volume of gas gives an amount, and combining that amount with a measured mass gives a molar mass. Each conversion and the rearrangement carry their own marks, so an arithmetic slip at the end still leaves most of them available.

    What earns the marks

    • The amount in moles obtained from mass divided by molar mass.
    • Converting both the temperature and the pressure into SI units.
    • The correct rearrangement of the gas equation for the required quantity.
    • The final answer converted into the unit demanded by the question.

    Common mistakes and how to put them right

    • Leaving the answer in cubic metres when cubic centimetres were requested.

    • Rounding the amount in moles to one or two significant figures early and carrying the error through.

    • Using the gas constant in joules alongside a pressure left in kilopascals.

    • Using the molar mass of the wrong species, for example the product rather than the gas measured.

    Exam tips

    • Show the substituted equation in full; substitution marks are awarded even when the final value is wrong.
    • Convert the answer to the requested unit as a separate final line so it is clearly visible.
    • Sense-check the result: roughly one mole of any gas occupies about 24 dm³ near room temperature and pressure.

    Try the lesson

    The start of the The ideal gas equation: Students should be able to: use the equation in calculations. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which rearrangement of the ideal gas equation pV = nRT gives the amount of gas n?

    Listen

    A typical calculation chains the gas equation to a mole calculation. Given a mass of propanone, first find the amount using n = mass ÷ molar mass. Then convert the data: 95 °C becomes 368 K and 103 kPa becomes 103000 Pa. Rearranging gives V = nRT/p, which returns a volume in cubic metres; multiplying by 10⁶ converts to the cubic centimetres the question wanted, about 75 cm³. Working the other way, a measured volume of gas gives an amount, and combining that amount with a measured mass gives a molar mass. Each conversion and the rearrangement carry their own marks, so an arithmetic slip at the end still leaves most of them available. A strong answer earns marks for these points. the amount in moles obtained from mass divided by molar mass. converting both the temperature and the pressure into SI units. the correct rearrangement of the gas equation for the required quantity. the final answer converted into the unit demanded by the question. Exam tip: Show the substituted equation in full; substitution marks are awarded even when the final value is wrong.

    See it

    A flowchart running top to bottom. The first path starts with a node labelled 'Mass of gas given', which leads to 'Find amount: n = mass ÷ molar mass', then to 'Convert temperature to K and pressure to Pa', then to 'Rearrange gas equation for required quantity', then to 'Calculate answer', and finally to 'Convert answer to unit demanded, e.g. m³ to cm³'. A second path starts with 'Measured volume of gas', leads to 'Find amount from gas equation', and then to 'Combine amount with measured mass to give molar mass'.
    A sequence showing how a gas calculation chains the gas equation to a mole calculation, and how the reverse route gives a molar mass.

    Key words

    Say what each word means before you turn the card.

    moles from mass

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    unit conversion at the end

    Try answering aloud or in your head before turning the card.

    rearranging the gas equation

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    See every definition
    moles from mass
    The amount is found by dividing the mass by the molar mass.
    unit conversion at the end
    The final answer is converted into the unit demanded by the question, such as multiplying by 10⁶ to change cubic metres to cubic centimetres.
    rearranging the gas equation
    The gas equation is rearranged for the required quantity, giving V = nRT/p for a volume.

    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 flowchart running top to bottom. The first path starts with a node labelled 'Mass of gas given', which leads to 'Find amount: n = mass ÷ molar mass', then to 'Convert temperature to K and pressure to Pa', then to 'Rearrange gas equation for required quantity', then to 'Calculate answer', and finally to 'Convert answer to unit demanded, e.g. m³ to cm³'. A second path starts with 'Measured volume of gas', leads to 'Find amount from gas equation', and then to 'Combine amount with measured mass to give molar mass'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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