O Levels Chemistry (5070)•5070/12/O/N/23
Explanation
Mass percentage of chromium in chrome alum KCr(SO₄)₂
Steps:
- Identify formula as KCr(SO₄)₂ (chrome alum's anhydrous form, with Cr implied in "KSO₄").
- Calculate molar mass: K (39) + Cr (52) + 2S (64) + 8O (128) = 283 g/mol.
- Mass of Cr = 52 g/mol.
- % Cr = (52 / 283) × 100 ≈ 18.4%.
Why B is correct:
- Matches calculation using standard atomic masses in the formula KCr(SO₄)₂.
Why the others are wrong:
- A: Underestimates by using incorrect molar mass or omitting Cr.
- C: Overestimates, possibly from hydrated form miscalculation.
- D: Further overestimates, ignoring full sulfate contribution.
Final answer: B
Topic: Relative masses of atoms and molecules
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