GCSE Chemistry (AQA)

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Mole Calculations

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Quantitative chemistry Use of amount of substance in relation to masses of pure substances

Mole Calculations

11:43 Moles, Mass, and Balanced Equations
Spec 4.3.2.1
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  • The number of moles in a substance can be calculated using its mass in grams and relative formula mass.
  • The formula is: Moles (N) = Mass (M) / Relative Formula Mass (MR).
  • For exams, you need to use and rearrange this formula.
  • A formula triangle can help: N (moles) at the bottom left, M (mass) on top, MR (relative formula mass) at the bottom right.
  • Mass should always be in grams for consistency.
  • Example: Calculate moles in 80 grams of water using MR of 18, resulting in 4.44 moles.
  • Example: Calculate moles in 0.5 kg of NH3 by converting mass to grams and using MR of 17, resulting in 29.4 moles.
  • MR can be calculated from mass and moles, useful for identifying unknown compounds.
  • Rearrange the formula to MR = M / N.
  • Example: Calculate MR of a compound with 0.82 grams and 0.01 moles, resulting in 82 grams per mole.
  • Mass can be calculated from MR and moles using the rearranged formula M = N * MR.
  • Example: Calculate mass of 6 moles of nitrogen with MR of 28, resulting in 168 grams.
  • Example: Calculate mass of 3 moles of PH3 with MR of 34, resulting in 102 grams.
  • Be comfortable with rearranging the formula to calculate moles, mass, or relative formula mass.

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