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Percentage Composition by Mass

Sometimes it is useful to know the percentage composition by mass of an element in a compound.For example nitrogen is needed for plant growth and is added to the soil in fertilisers.One of the main fertilisers used is ammonium nitrate NH₄NO₃.The percentage composition by mass of nitrogen in ammonium nitrate can be calculated by dividing the relative atomic mass of nitrogen × the number of nitrogen atoms in one formula unit of ammonium nitrate by the relative formula mass of ammonium nitrate and multiplying by 100% to give a percentage.

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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

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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