Percentage Composition by Mass

This lesson teaches you how to calculate the percentage by mass of an element in a compound. This is a common calculation in GCSE Chemistry and is required by the Edexcel specification (1CH0).

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The Formula

To calculate the percentage by mass of an element in a compound:

$\text{percentage by mass} = \frac{A_r \times \text{number of atoms of that element}}{M_r \text{ of the compound}} \times 100$percentage by mass=Mr​ of the compoundAr​×number of atoms of that element​×100

Or more simply:

$\% = \frac{\text{total mass of the element in the formula}}{M_r \text{ of the compound}} \times 100$%=Mr​ of the compoundtotal mass of the element in the formula​×100

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Step-by-Step Method

1. Calculate Mr of the whole compound.
2. Find the total mass of the element you are interested in (Ar × number of atoms of that element in the formula).
3. Divide the total mass of the element by Mr.
4. Multiply by 100 to get the percentage.

Exam Tip: Always show your Mr calculation clearly. This earns marks and helps you avoid errors.

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Worked Examples

Example 1: Percentage of Oxygen in Water (H₂O)

Step 1: Mr of H₂O = (2 × 1) + 16 = 18

Step 2: Total mass of oxygen = 1 × 16 = 16