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AQA · GCSE · Chemistry · Higher

Percentage iron by mass in magnetite

Q07.5 3 marks

Relative atomic masses (Aᵣ): O = 16, Fe = 56.

Calculate the percentage (%) by mass of iron in Fe₃O₄.

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Marking points

  1. 1 Determines the formula mass as 232.
  2. 2 Uses iron mass 168 over the formula mass and multiplies by 100.
  3. 3 Final percentage is 72.4%, or an accepted correctly rounded equivalent.

Full-mark answer

Mᵣ(Fe₃O₄) = (3 × 56) + (4 × 16) = 232. Percentage Fe = (3 × 56 ÷ 232) × 100 = 72.4%.

Why this answer loses marks

I divided the iron atomic mass by the oxygen atomic mass.

Find the whole formula mass and iron’s total contribution before converting the share to a percentage.

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