AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to calculate the percentage by mass in a compound…
Full specification wording
Relative formula mass: Students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses.
Percentage by mass shows the contribution of one element to the total mass of a compound.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 2
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Percentage by mass shows the contribution of one element to the total mass of a compound. First calculate the relative formula mass, Mᵣ, by summing all relative atomic masses in the formula. Then find the total mass of the element of interest by multiplying its relative atomic mass by the number of its atoms in the formula. Divide that element's total mass by Mᵣ and multiply by 100. For example, in CO₂, Mᵣ = 12 + (2 × 16) = 44; carbon contributes 12 ÷ 44 × 100 = 27.3% and oxygen contributes 32 ÷ 44 × 100 = 72.7%. The percentages should sum to 100%, which is a useful check.
What earns the marks
- Calculate Mᵣ by summing the relative atomic masses of all atoms in the formula.
- Find the total mass of the chosen element using its relative atomic mass and atom count.
- Divide the element's total mass by Mᵣ and multiply by 100 to obtain percentage by mass.
- Worked example: in CO₂, carbon is 12 ÷ 44 × 100 = 27.3% and oxygen is 32 ÷ 44 × 100 = 72.7%.
- The percentages of all elements in a compound should sum to 100%.
- Give answers to an appropriate number of significant figures, typically three.
Common mistakes and how to put them right
Dividing by the element's mass instead of Mᵣ.
Put it right: Divide the element's total mass by the compound's Mᵣ.
Forgetting to multiply the relative atomic mass by the number of atoms of that element.
Put it right: Include the atom count, for example 2 × 16 for oxygen in CO₂.
Rounding intermediate values too early, causing a final answer that is slightly wrong.
Put it right: Keep full values during the calculation and round only at the end.
Exam tips
- Write down Mᵣ first and label it clearly before starting the percentage calculation.
- Show the numerator as the element's total mass and the denominator as Mᵣ, then multiply by 100.
- Check that the percentages for all elements add up to 100% as a final verification.
- State the element and the compound in your answer, for example 'percentage by mass of carbon in CO₂'.
Try the lesson
The start of the Relative formula mass: Students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses. lesson, made from checked teaching notes for this specification point.
Have a go first
In carbon dioxide, CO₂, the relative formula mass is 44 and the relative atomic mass of carbon is 12. What is the percentage by mass of carbon in carbon dioxide?
Listen
Percentage by mass shows the contribution of one element to the total mass of a compound. First calculate the relative formula mass, Mᵣ, by summing all relative atomic masses in the formula. Then find the total mass of the element of interest by multiplying its relative atomic mass by the number of its atoms in the formula. Divide that element's total mass by Mᵣ and multiply by 100. For example, in CO₂, Mᵣ = 12 + (2 × 16) = 44; carbon contributes 12 ÷ 44 × 100 = 27.3% and oxygen contributes 32 ÷ 44 × 100 = 72.7%. The percentages should sum to 100%, which is a useful check. A strong answer earns marks for these points. Calculate Mᵣ by summing the relative atomic masses of all atoms in the formula. Find the total mass of the chosen element using its relative atomic mass and atom count. Divide the element's total mass by Mᵣ and multiply by 100 to obtain percentage by mass. Worked example: in CO₂, carbon is 12 ÷ 44 × 100 = 27.3% and oxygen is 32 ÷ 44 × 100 = 72.7%. A common slip: Dividing by the element's mass instead of Mᵣ. Instead, divide the element's total mass by the compound's Mᵣ. Exam tip: Write down Mᵣ first and label it clearly before starting the percentage calculation.
See it
Put it in order
Put these steps in the order they happen.
- 1. Find total mass of the element: relative atomic mass × number of its atoms
- 2. Divide the element's total mass by Mᵣ
- 3. Calculate Mᵣ by summing all relative atomic masses in the formula
- 4. Check that the percentages of all elements sum to 100%
- 5. Multiply by 100 to obtain percentage by mass
Key words
Say what each word means before you turn the card.
Relative formula mass
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Percentage by mass
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Percentage composition
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See every definition
- Relative formula mass
- The sum of all relative atomic masses in the formula, written as Mᵣ.
- Percentage by mass
- The contribution of one element to the total mass of a compound, found by dividing the element's total mass by Mᵣ and multiplying by 100.
- Percentage composition
- The percentages of all elements in a compound, which should sum to 100% as a useful check.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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