AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The ideal gas equation pV = nRT with the variables in SI units.

    Full specification wording

    The ideal gas equation: The ideal gas equation pV = nRT with the variables in SI units.

    The ideal gas equation is pV = nRT, and it only works when every quantity is in base SI units.

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

    • Point 1 of 2

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The ideal gas equation is pV = nRT, and it only works when every quantity is in base SI units. Pressure must be in pascals, so kilopascals are multiplied by 1000. Volume must be in cubic metres, so cubic centimetres are divided by 10⁶ and cubic decimetres by 1000. Temperature must be in kelvin, so add 273 to a Celsius value. The gas constant R is 8.31 J K⁻¹ mol⁻¹, and its units are the check: joules per kelvin per mole only cancel correctly when pressure is in pascals and volume in cubic metres. The equation applies to an ideal gas, in which the particles have negligible volume and no forces between them.

    What earns the marks

    • Converting a temperature in degrees Celsius into kelvin by adding 273.
    • Converting a pressure in kilopascals into pascals.
    • Converting a volume into cubic metres before substituting.
    • A correct rearrangement of pV = nRT for the quantity required.

    Common mistakes and how to put them right

    • Substituting a temperature in degrees Celsius.

    • Using kilopascals with a gas constant expressed in joules.

    • Dividing a volume in cubic centimetres by 1000 instead of by 10⁶.

    • Reporting a volume in cubic metres when the question asked for cubic centimetres.

    Exam tips

    • List p, V, n, R and T down the page with their converted values before substituting anything.
    • Use the units of R printed in the question to confirm which units each other quantity needs.
    • Rearrange the equation symbolically first, then substitute; it is easier to spot an inverted fraction that way.

    Try the lesson

    The start of the The ideal gas equation: The ideal gas equation pV = nRT with the variables in SI units. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which set of SI units is correct for the variables in the ideal gas equation pV = nRT?

    Listen

    The ideal gas equation is pV = nRT, and it only works when every quantity is in base SI units. Pressure must be in pascals, so kilopascals are multiplied by 1000. Volume must be in cubic metres, so cubic centimetres are divided by 10⁶ and cubic decimetres by 1000. Temperature must be in kelvin, so add 273 to a Celsius value. The gas constant R is 8.31 J K⁻¹ mol⁻¹, and its units are the check: joules per kelvin per mole only cancel correctly when pressure is in pascals and volume in cubic metres. The equation applies to an ideal gas, in which the particles have negligible volume and no forces between them. A strong answer earns marks for these points. converting a temperature in degrees Celsius into kelvin by adding 273. converting a pressure in kilopascals into pascals. converting a volume into cubic metres before substituting. a correct rearrangement of pV = nRT for the quantity required. Exam tip: List p, V, n, R and T down the page with their converted values before substituting anything.

    See it

    A flowchart starting from the node pV = nRT, which branches to four nodes: Pressure in pascals, Volume in cubic metres, Temperature in kelvin, and R = 8.31 J K⁻¹ mol⁻¹. Pressure in pascals leads to kPa × 1000. Volume in cubic metres leads to two nodes: cm³ ÷ 10⁶ and dm³ ÷ 1000. Temperature in kelvin leads to °C + 273. R = 8.31 J K⁻¹ mol⁻¹ leads to a node reading Units of R check the other units.
    It shows the ideal gas equation with the SI unit required for each quantity and how each given value is converted.

    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 starting from the node pV = nRT, which branches to four nodes: Pressure in pascals, Volume in cubic metres, Temperature in kelvin, and R = 8.31 J K⁻¹ mol⁻¹. Pressure in pascals leads to kPa × 1000. Volume in cubic metres leads to two nodes: cm³ ÷ 10⁶ and dm³ ÷ 1000. Temperature in kelvin leads to °C + 273. R = 8.31 J K⁻¹ mol⁻¹ leads to a node reading Units of R check the other units.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of The ideal gas equation: The ideal gas equation pV = nRT with the variables in SI units.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.