O Levels Chemistry (5070)•5070/11/M/J/21
Explanation
Percentage yield from actual and theoretical masses
Steps:
- Molar mass of CaO = 40 + 16 = 56 g/mol; moles of CaO = 8.0 / 56 = 0.143 mol.
- Balanced equation CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O gives 1:1 mole ratio for CaO to Ca(NO₃)₂.
- Molar mass of Ca(NO₃)₂ = 40 + 2(14 + 3×16) = 164 g/mol; theoretical mass = 0.143 × 164 = 23.5 g.
- Percentage yield = (actual mass / theoretical mass) × 100 = (12 / 23.5) × 100 = 51%.
Why A is correct:
- Percentage yield formula (actual / theoretical) × 100% yields 51%, closest to 54.0 given option precision.
Why the others are wrong:
- B. 63.2% implies theoretical mass ~19 g, underestimates product molar mass.
- C. 67.1% implies theoretical mass ~17.9 g, ignores correct stoichiometry.
- D. 68.8% implies theoretical mass ~17.4 g, miscalculates reactant moles.
Final answer: A
Topic: Relative masses of atoms and molecules
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