More practice questions

4. Q4. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen, and its molar mass is 56 g mol^⁻¹. Find its molecular formula (C = 12.01, H = 1.008).

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   Moles in 100 g: C = 85.7/12.01 = 7.14; H = 14.3/1.008 = 14.2. Ratio 1 : 2, so the empirical formula is CH₂ (14.03 g mol^⁻¹). n = 56/14.03 ≈ 4, so the molecular formula is C₄H₈.

5. Q5. NCERT Exercise 1.8 (page 26) asks for the molecular formula of the iron oxide with 69.9% iron and 30.1% oxygen. What extra information is needed, and what is the answer if the molar mass is 159.7 g mol^⁻¹?

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   The percentages give only the empirical formula, Fe₂O₃ (Example 3). The molar mass is also needed. Fe₂O₃ has formula mass 2 × 55.85 + 3 × 16.00 = 159.70 g mol^⁻¹, so with a molar mass of 159.7 g mol^⁻¹, n = 1. Iron(III) oxide is ionic, so its formula is the formula unit, Fe₂O₃, the same as its empirical formula; this is the answer NCERT expects.

6. Q6. Pure calcium carbonate, CaCO₃, is 40.04% calcium by mass (Ca = 40.08, C = 12.01, O = 16.00). A sample of impure limestone contains 36.0% calcium, and its impurities contain no calcium. What is the percentage of CaCO₃ in the sample?

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   Each gram of CaCO₃ brings 0.4004 g of calcium, so the sample is 36.0/40.04 × 100 ≈ 89.9% CaCO₃.