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Percentage CompositionOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is percentage composition?

Percentage composition describes the proportion, by mass, of each element in a compound.

It tells you what percentage of the total mass of the compound is contributed by a particular element.

This is useful for comparing compounds, checking purity, and choosing raw materials (for example, deciding which iron ore gives the highest yield of iron).

Key termspercentage composition

Section 2

How do you calculate the percentage by mass of an element?

Use the equation:

% by mass = (Ar × number of atoms of that element) ÷ Mr × 100

Steps:

  1. Find the relative atomic mass (Ar) of the element
  2. Multiply by the number of atoms of that element in the formula
  3. Divide by the relative formula mass (Mr) of the whole compound
  4. Multiply by 100 to convert to a percentage
Key termsrelative atomic mass (Ar)relative formula mass (Mr)
Example

Calculate the percentage by mass of carbon in carbon dioxide, CO2 (Ar: C = 12, O = 16). Mr(CO2) = 12 + (16 × 2) = 44. % C = (12 × 1) ÷ 44 × 100 = 27.3%.

Section 3

How do you work with compounds containing more than two elements?

The same method applies, but you must be careful to count the correct number of atoms of the element in question from the formula.

For compounds with brackets, such as Ca(OH)2, remember the bracket multiplies everything inside it — so there are 2 oxygen atoms and 2 hydrogen atoms.

Example

Calculate the percentage by mass of oxygen in calcium hydroxide, Ca(OH)2 (Ar: Ca = 40, O = 16, H = 1). Mr = 40 + 2(16+1) = 74. % O = (16 × 2) ÷ 74 × 100 = 43.2%.

Exam tip

Always calculate Mr first as a separate step and label it clearly — this is often worth its own mark even if the final percentage is wrong.

Section 4

Why is percentage composition useful?

Percentage composition calculations are used to:

  • Compare the proportion of a useful element (e.g. a metal) in different compounds or ores
  • Check the purity of a sample by comparing the expected and measured percentage of an element
  • Work out the maximum mass of a product obtainable from a given mass of raw material

Must Know

  • % by mass = (Ar × number of atoms of the element) ÷ Mr × 100
  • Always calculate Mr of the whole compound first
  • Brackets in a formula multiply every atom inside them by the number outside
  • Percentage composition is used to compare ores, check purity, and predict yields
  • The percentages of all elements in a compound must add up to 100%

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