Page 2. Q1.Water dissociates slightly according to the equation: H 2 O(I) The ionic product of water, K w, is given by the expression
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1 Q1.Water dissociates slightly according to the equation: H 2 O(I) H + (aq) + OH (aq) The ionic product of water, K w, is given by the expression K w = [H + ][OH ] K w varies with temperature as shown in the table. Temperature / C K w / mol 2 dm (a) Explain why the expression for K w does not include the concentration of water. (b) Explain why the value of K w increases as the temperature increases. (c) Calculate the ph of pure water at 50 C. Page 2
2 (d) Calculate the ph of 0.12 mol dm 3 aqueous NaOH at 50 C. (Total 10 marks) Q2.Ammonium chloride, when dissolved in water, can act as a weak acid as shown by the following equation. NH 4+ (aq) NH 3(aq) + H + (aq) The following figure shows a graph of data obtained by a student when a solution of sodium hydroxide was added to a solution of ammonium chloride. The ph of the reaction mixture was measured initially and after each addition of the sodium hydroxide solution. Page 3
3 (a) Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution. Explain why this apparatus is more suitable than a pipette for this purpose. Apparatus... Explanation... (b) Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator. Page 4
4 (c) The ph at the end point of this reaction is Use this ph value and the ionic product of water, K w = mol 2 dm 6, to calculate the concentration of hydroxide ions at the end point of the reaction. Concentration =... mol dm 3 (d) The expression for the acid dissociation constant for aqueous ammonium ions is The initial concentration of the ammonium chloride solution was 2.00 mol dm 3. Use the ph of this solution, before any sodium hydroxide had been added, to calculate a value for K a K a =... mol dm 3 (e) A solution contains equal concentrations of ammonia and ammonium ions. Use your value of K a from part (d) to calculate the ph of this solution. Explain your working. (If you were unable to calculate a value for K a you may assume that it has the value mol dm 3. This is not the correct value.) Page 5
5 ph=... (Total 12 marks) Q3.This question involves calculations about two strong acids and one weak acid. All measurements were carried out at 25 C. (a) A 25.0 cm 3 sample of mol dm 3 hydrochloric acid was placed in a beaker and 100 cm 3 of distilled water were added. Calculate the ph of the new solution formed. (Extra space)... (b) HX is a weak monobasic acid. (i) Write an expression for the acid dissociation constant, K a, for HX. (1) (ii) The ph of a mol dm 3 solution of HX is 2.79 Calculate a value for the acid dissociation constant, K a, of this acid. Give your answer to 3 significant figures. Page 6
6 (Extra space)... (c) A 25.0 cm 3 sample of mol dm 3 nitric acid was placed in a beaker and 38.2 cm 3 of mol dm 3 aqueous sodium hydroxide were added. Calculate the ph of the solution formed. The ionic product of water K w = mol 2 dm 6 at 25 C. (Extra space)... Page 7
7 (6) (Total 12 marks) Q4.What is the ph of a mol dm 3 solution of a diprotic acid which is completely dissociated? A 1.00 B 1.40 C 1.70 D 4.00 (Total 1 mark) Q5.This question is about alkalis and carboxylic acids. In this question, all data are quoted at 25 C. (a) Carboxylic acids are weak acids. State the meaning of the term weak as applied to carboxylic acids. (1) (b) Write an equation for the reaction of propanoic acid with sodium carbonate. (1) Page 8
8 (c) Calculate the ph of a mol dm 3 solution of calcium hydroxide. The ionic product of water K w = mol 2 dm 6. (Extra space)... (d) The value of the acid dissociation constant K a for benzenecarboxylic acid (C 6H 5COOH) is mol dm 3. (i) Write an expression for the acid dissociation constant K a for benzenecarboxylic acid. (1) (ii) Calculate the ph of a mol dm 3 solution of benzenecarboxylic acid. Page 9
9 (Extra space)... (iii) A buffer solution with a ph of 4.00 is made using benzenecarboxylic acid and sodium benzenecarboxylate. Calculate the mass of sodium benzenecarboxylate (M r = 144.0) that should be dissolved in 1.00 dm 3 of a mol dm 3 solution of benzenecarboxylic acid to produce a buffer solution with a ph of 4.00 The value of the acid dissociation constant K a for benzenecarboxylic acid (C 6H 5COOH) is mol dm 3. (Extra space)... (5) (e) Two solutions, one with a ph of 4.00 and the other with a ph of 9.00, were left open to the air. Page 10
10 The ph of the ph 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the ph to change. Explain how and why the ph of the ph 9.00 solution changes. Substance present in air... Explanation... (Total 17 marks) Page 11
Page 2. Q1.Water dissociates slightly according to the equation: H 2 O(I) The ionic product of water, K w, is given by the expression
Q1.Water dissociates slightly according to the equation: H 2 O(I) H + (aq) + OH (aq) The ionic product of water, K w, is given by the expression K w = [H + ][OH ] K w varies with temperature as shown in
NH 3(aq) + H + (aq)
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