O Levels Chemistry (5070)•5070/11/O/N/21
Explanation
Percentage by mass of nitrogen in ammonium sulphate and nitrate Steps:
- Identify chemical formulas: ammonium sulphate is (NH₄)₂SO₄; ammonium nitrate is NH₄NO₃.
- Calculate molar mass of (NH₄)₂SO₄: (2×14) + (8×1) + 32 + (4×16) = 132 g/mol.
- Calculate %N in (NH₄)₂SO₄: (28/132) × 100 = 21.2%.
- Calculate %N in NH₄NO₃: molar mass = 80 g/mol, N mass = 28 g/mol, (28/80) × 100 = 35.0%. Why B is correct:
- Provides accurate %N values matching the formula for percentage composition: (mass of N atoms / molar mass of compound) × 100. Why the others are wrong:
- A: Both values incorrect—10.6% underestimates N in sulphate (wrong formula or atomic masses); 17.5% underestimates N in nitrate.
- C: 21.2% correct for sulphate, but 17.5% wrong for nitrate (counts only one N atom).
- D: Same as C, so incorrect for nitrate. Final answer: B
Topic: Relative masses of atoms and molecules
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