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AQA GCSE · Question 07.5 · Quantitative chemistry
Calculate the percentage (%) by mass of iron in Fe₃O₄.
Relative atomic masses (Aᵣ): O = 16, Fe = 56
Calculate the percentage (%) by mass of iron in Fe₃O₄.
Relative atomic masses (Aᵣ): O = 16, Fe = 56
How to approach this question
1. Calculate the relative formula mass (Mᵣ) of the whole compound, Fe₃O₄. Remember to multiply the Aᵣ of each element by the number of atoms in the formula.
2. Calculate the total mass of just the iron atoms in the formula.
3. Use the formula for percentage by mass:
% mass = (total mass of element / Mᵣ of compound) × 100.
4. Substitute your values and calculate the final answer.
Full Answer
1. Calculate the relative formula mass (Mᵣ) of Fe₃O₄:
Mᵣ(Fe₃O₄) = (3 × Aᵣ(Fe)) + (4 × Aᵣ(O))
Mᵣ(Fe₃O₄) = (3 × 56) + (4 × 16)
Mᵣ(Fe₃O₄) = 168 + 64 = 232
2. Calculate the total mass of iron in the formula:
Mass of Fe = 3 × 56 = 168
3. Calculate the percentage by mass of iron:
% Fe = (Total mass of Fe / Mᵣ of Fe₃O₄) × 100
% Fe = (168 / 232) × 100
% Fe = 72.413... %
% Fe ≈ 72.4 % (to 3 s.f.)
To calculate the percentage by mass of an element in a compound, we use the following steps:
**Step 1: Calculate the relative formula mass (Mᵣ) of the compound.**
The formula is Fe₃O₄.
Aᵣ of Fe = 56
Aᵣ of O = 16
Mᵣ(Fe₃O₄) = (Number of Fe atoms × Aᵣ of Fe) + (Number of O atoms × Aᵣ of O)
Mᵣ(Fe₃O₄) = (3 × 56) + (4 × 16)
Mᵣ(Fe₃O₄) = 168 + 64 = 232
**Step 2: Calculate the total mass of the element in question within the formula.**
Total mass of iron (Fe) = 3 × 56 = 168
**Step 3: Calculate the percentage by mass.**
% by mass = (Total mass of element / Mᵣ of compound) × 100
% Fe = (168 / 232) × 100
% Fe = 0.72413... × 100
% Fe = 72.413... %
Rounding to three significant figures gives **72.4 %**.
Common mistakes
✗ Calculating the Mᵣ incorrectly (e.g., forgetting to multiply by the number of atoms).
✗ Dividing the Mᵣ by the mass of iron instead of the other way around.
✗ Forgetting to multiply by 100.
Practice the full AQA GCSE Chemistry Higher Tier Paper 1
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