16 7 This question is about acids and buffer solutions. (a) Ethanoic acid, CH3COOH, is a monobasic acid. CH3COOH + H2O  CH3COO– + H3O+ Give a reason why only the proton from the carboxylic acid group, and not from the methyl group, is donated to a water molecule. 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(b) The reaction of ammonia with water can be represented by NH3 + H2O  NH4 + + OH– Which is the acid‑conjugate base pair? (1) Acid Conjugate base A NH3 OH– B NH3 NH4 + C H2O OH– D H2O NH4 + Turn over 17 (c) A commercial nitric acid solution, HNO3(aq), has a concentration of 15.9 mol dm–3. A 15.0 cm3 sample was made up to 100 cm3 by adding deionised water. Calculate the pH of this diluted solution. (2) 18 (d) Propanoic acid is a weak acid. (i) Calculate the pH of a 0.100 mol dm–3 solution of propanoic acid at 298 K. Give your answer to an appropriate number of significant figures. [Ka = 1.35 × 10–5 mol dm–3 at 298 K] (3) (ii) State two assumptions that you made in the calculation in d(i). 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Turn over 19 (e) A buffer solution was made using 20.0 cm3 of a butanoic acid solution, of concentration 0.100 mol dm–3 and 30.0 cm3 of sodium butanoate solution, of concentration 0.305 mol dm–3. [Ka = 1.52 × 10–5 mol dm–3 at 298K] (i) Calculate the pH of this buffer solution at 298 K. (4) (ii) Explain why the pH of the buffer solution hardly changes when a few drops of sodium hydroxide solution are added to it. Include an equation or equations in your answer. Use C3H7COOH as the formula for butanoic acid. 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(Total for Question 7 = 15 marks)