OCR A Jun 2014 Paper 5 Q6

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6 Ethanoic acid, CH3COOH, is a weak BrnstedLowry acid. 13 (a) An acidbase equilibrium is set up when ethanoic acid is added to water. Write the equation for the equilibrium that would be set up and label the two conjugate acidbase pairs.++[2] (b) An aqueous solution of CH3COOH has a pH of 3.060. This solution contains both hydrogen ions and hydroxide ions. (i) How can an aqueous solution of an acid contain hydroxide ions?[1] (ii) Calculate the concentration of hydroxide ions in this solution of ethanoic acid. concentration of hydroxide ions =mol dm3 [2]OCR 2014 Turn over<br />
 (c) A student adds an excess of aqueous ethanoic acid to solid calcium carbonate. The resulting solution is able to act as a buffer solution. 14 (i) Write a full equation for the reaction between ethanoic acid and solid calcium carbonate.[1] (ii) Explain why this buffer solution has formed.[1] (iii) Explain how this buffer solution controls pH when either an acid or an alkali is added. In your answer you should explain how the equilibrium system allows the buffer solution to control the pH.[5]OCR 2014<br />
 15 (d) A biochemist plans to make up a buffer solution with a pH of 5.000. The biochemist adds solid sodium ethanoate, CH3COONa, to 400 cm3 of 0.200 mol dm3 ethanoic acid. Ka for ethanoic acid = 1.75105 mol dm3 Calculate the mass of sodium ethanoate that the biochemist needs to dissolve in the ethanoic acid to prepare this buffer solution. Assume that the volume of the solution remains constant at 400 cm3 on dissolving the sodium ethanoate.OCR 2014 [5] [Total: 17] Turn over<br />

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