More practice questions

3. Q3. Which of ethanoic acid, phenol and ethanol turn moist blue litmus red, and which give effervescence with sodium hydrogencarbonate?

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   Litmus: by the Lab Manual’s reading (page 96), ethanoic acid and phenol, though ordinary phenol, a weak acid, may not turn blue litmus distinctly red at the tested concentration (reasoned). Effervescence: only ethanoic acid among these three (Lab Manual, page 95). Ethanol, a far weaker acid (pK_a about 16, NCERT page 250), does neither (reasoned).

4. Q4. Write the equation for the ester test on benzoic acid with methanol, and name the product.

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   C₆H₅COOH + CH₃OH ⇌ C₆H₅COOCH₃ + H₂O (conc. H₂SO₄, heat); the ester is methyl benzoate (reasoned from Lab Manual, page 96, and NCERT page 251).

5. Q5. What mass of sodium hydrogencarbonate reacts with 0.60 g of ethanoic acid? (C = 12, H = 1, O = 16, Na = 23.)

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   M(CH₃COOH) = 60 g mol⁻¹, so 0.60 g is 0.010 mol; it reacts 1 : 1 with NaHCO₃, M = 23 + 1 + 12 + 48 = 84 g mol⁻¹: 0.010 × 84 = 0.84 g (reasoned).

6. Q6. Why must the litmus paper be moist?

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   The acid has to ionise to change the colour, and it needs water to do so (reasoned; the Lab Manual, page 96, asks for moist blue litmus).