More practice questions

4. Q4. Urea, CO(NH₂)₂, is a common nitrogen fertiliser. Find its mass per cent of nitrogen, using C = 12.01, O = 16.00, N = 14.01 and H = 1.008 u.

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   Molar mass = 12.01 + 16.00 + 2 × 14.01 + 4 × 1.008 = 60.062 g mol^⁻¹. Nitrogen = 28.02/60.062 × 100 ≈ 46.65%.

5. Q5. Find the mass per cent of carbon in methane, CH₄, a greenhouse gas (C = 12.01, H = 1.008 u).

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   Molar mass = 12.01 + 4 × 1.008 = 16.042 g mol^⁻¹. Carbon = 12.01/16.042 × 100 ≈ 74.87%.

6. Q6. Why does NCERT call chemistry the science of atoms and molecules?

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   Chemistry deals with the composition, structure, properties and interactions of matter, and all of these are best described and understood in terms of the basic constituents of matter, atoms and molecules (NCERT page 3).