MCQ practice · SSC Chemistry Board questions · Chapter 6: Concept of Mole and Chemical Counting
10 g of calcium completes its reaction with how many grams of oxygen?
Think it through first, then pick an option.
- a 32
- b 16
- c 8
- d 4
Answer and explanation
Correct answer: d 4
2Ca + O₂ → 2CaO: 80 g Ca ↔ 32 g O₂, so 10 g Ca ↔ 32 × 10/80 = 4 g O₂.
Why the other options are wrong
- a 32: 32 g of O₂ serves 80 g of Ca (2Ca + O₂ → 2CaO); 10 g of Ca needs 4 g.
- b 16: 16 g of O₂ serves 40 g of Ca; 10 g of Ca needs 4 g.
- c 8: 8 g of O₂ serves 20 g of Ca; 10 g needs half, 4 g.
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