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Calculations

The mass of a sample of substance can be used to count the number of atoms in the sample by using the formula moles = mass ÷ M_rwhere M_r is the relative atomic mass or relative molecular mass or relative formula mass of the substance.This is important since substances react by the recombination of atoms and the number of atoms and mass is conserved in any chemical reaction.In order to calculate the amount of substance that has reacted and the amount of product formed you need to calculate the number of moles of the reactants and products.

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Specification: Edexcel Chemistry (8CH0), AS-level, Topic 5: Formulae, Equations and Amounts of Substance. Topic: Amount of Substance and Calculations.

Key terms in this lesson

mole: The amount of substance containing 6.02 x 10^23 particles, the same number as the atoms in 12 g of carbon-12.
relative atomic mass: The weighted mean mass of one atom of an element compared with 1/12 of the mass of one atom of carbon-12.
relative molecular mass: The sum of the relative atomic masses of the atoms in a molecule's formula, compared with 1/12 the mass of carbon-12.

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