QUESTIONSHEETS ACID - BASE EQUILIBRIA. ph OF STRONG ACIDS AND BASES. AND pk a. ph OF ACID OR BASE SOLUTIONS. ph OF MIXED SOLUTIONS OF ACID AND BASE

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1 CHEMISTRY QUESTIONSHEETS A2 Level A2 TOPIC 19 ACID - BASE EQUILIBRIA Questionsheet 1 Questionsheet 2 Questionsheet 3 Questionsheet 4 Questionsheet 5 Questionsheet 6 Questionsheet 7 Questionsheet 8 Questionsheet 9 Questionsheet 10 Questionsheet 11 Questionsheet 12 Questionsheet 13 ph and K w ph OF STRONG ACIDS AND BASES STRENGTHS OF ACIDS (K a AND pk a ) ph OF ACID OR BASE SOLUTIONS ph OF MIXED SOLUTIONS OF ACID AND BASE BRØNSTED-LOWRY THEORY I THEORY OF INDICATORS TITRATION CURVES ACID-BASE STRENGTH AND ENTHALPY OF NEUTRALISATION BUFFER SOLUTIONS ph OF BUFFER SOLUTIONS TEST QUESTION I TEST QUESTION II 17 marks 15 marks 14 marks 19 marks 14 marks 15 marks 16 marks 16 marks 15 marks 19 marks 13 marks 16 marks 14 marks Authors Trevor Birt Donald E Caddy Andrew Jones Adrian Bond Editors John Brockington Stuart Barker John Brockington Kevin Frobisher Andy Shepherd Stuart Barker Curriculum Press Licence Agreement: Paper copies of the A-Level Chemistry Questionsheets may be copied free of charge by teaching staff or students for use within their school, provided the Photocopy Masters have been purchased by their school. No part of these Questionsheets may be reproduced or transmitted, in any other form or by any other means, without the prior permission of the publisher. All rights are reserved. This license agreement is covered by the laws of England and Wales Curriculum Press March Curriculum P ress Curriculum Press Bank House 105 King Street Wellington Shropshire TF1 1NU

2 a) Define the term ph. TOPIC 19 Questionsheet 1 ph and K w b) What is meant by the self-ionisation of water? Write an expression for the dissociation constant, and explain how it can be modified so as to define the ionic product (K w ) of water.... [4] c) K w has a numerical value of 1.00 x at 298 K. What are its units? d) At 373 K, K w has a numerical value of 5.14 x Calculate: (i) the concentration of hydrogen ions, [H + ]. the ph of water at this temperature. e) Is the dissociation of water endothermic or exothermic? Justify your answer by reference to the given values of K w.... [4] f) At 273 K, the ph of water is (i) Calculate K w at this temperature. Is water chemically neutral at this temperature? Explain your answer. TOTAL / 17

3 TOPIC 19 Questionsheet 2 ph OF STRONG ACIDS AND BASES a) Calculate the ph of the following solutions (i) mol dm -3 HNO mol dm -3 HCl (iii) mol dm -3 H 2 SO 4 (iv) mol dm -3 KOH (v) mol dm -3 Ba(OH) 2 b) An aqueous solution contains mol of HCl in 100 cm 3 of solution, hence calculate (i) the [H + ] the ph. c) What is the concentration in mol dm -3 of: (i) HCl(aq) which has a ph of 1.4? H 2 SO 4 (aq) which has a ph of 2.7? (iii) NaOH(aq) which has a ph of 11.3? TOTAL / 15

4 TOPIC 19 Questionsheet 3 PH astrengths OF ACIDS (K a AND pk a )nd pk w a) Propanoic acid, C 2 H 5 COOH, is a weak acid, with K a = 1.35 x 10-5 mol dm -3. (i) What is meant by the term weak acid? Write an expression for the acid dissociation constant, K a, of propanoic acid. (iii) Calculate the hydrogen ion concentration and ph of a solution of propanoic acid containing 0.20 mol dm -3. State what assumption is made in the calculation.... [4] b) Two acids, HS and HT, have pk a values of 3.6 and 4.1 respectively. (i) What is meant by the expression pk a? Calculate K a for HS and HT. HS... HT... [2] (iii) Which is the stronger of the two acids? Give your reason. c) The ammonium ion, NH 4+, can act as an acid. (i) Write an expression for the acid dissociation constant, K a, of the ammonium ion. [1] This K a has a value of 5.62 x mol dm -3. Calculate pk a. [1] d) Name of acid Formula pk a Butanoic CH 3 COOH Chlorobutanoic CH 3 CHClCOOH Chlorobutanoic CH 3 CHCl COOH Chlorobutanoic Cl COOH 4.52 Place these acids in order of increasing acid strength. TOTAL / 14

5 TOPIC 19 Questionsheet 4 ph OF ACID OR BASE SOLUTIONSK w a) Calculate the ph of each of the following acids and explain the difference in the values you obtain. (i) mol dm -3 nitric acid mol dm -3 butanoic acid (pk a = 4.82)... [4] Explanation... b) Calculate the ph of the resulting solutions when: (i) 10.0 cm 3 of l.0 mol dm -3 HCl are added to 990 cm 3 of water cm mol dm -3 NaOH are added to 980 cm 3 of water. c) (i) Write an expression for the acid dissociation constant, K a, of methanoic acid, HCOOH. The ph of a solution of methanoic acid, K a = 1.78 x 10-4 mol dm -3, is 4.0. Calculate the concentration of the acid. d) When mol of a weak acid HQ is dissolved in 500 cm 3 of water, the resultant ph is 3.0. Calculate the K a of the acid. TOTAL / 19 [1]

6 TOPIC 19 Questionsheet 5 ph OF MIXED SOLUTIONS OF ACID AND BASE a) Calculate the hydrogen ion concentration in each of the following solutions: (i) mol dm -3 HCl mol dm -3 ethanoic acid (pk a = 4.76). (iii) 100 cm 3 of 0.1 mol dm -3 HCl to which 50 cm 3 of mol dm -3 KOH have been added.... [4] b) Sulfuric acid is a strong dibasic acid, and reacts with potassium hydroxide solution as follows: H 2 SO 4 (aq) + 2KOH(aq) K 2 SO 4 (aq) + 2H 2 O(l) 20.0 cm 3 of sodium hydroxide solution of concentration mol dm -3 were added to 25.0 cm 3 of sulfuric acid of concentration mol dm -3. Calculate: (i) the moles of H + in the sulfuric acid. the moles of OH - added. (iii) the moles of H + unreacted. (iv) the ph of the resulting solution. TOTAL / 14

7 TOPIC 19 Questionsheet 6 BRØNSTED-LOWRY THEORY I a) (i) How does the BrØnsted-Lowry theory define an acid and a base? Acid... Base... [2] What do you understand by the terms conjugate acid and conjugate base? Conjugate acid... Conjugate base... [2] Quality of language [1] b) In each of the following reactions, identify the reacting species which is acting as an acid, and the product species which is the conjugate acid. Acid Conjugate acid (i) CH 3 COOH(aq) + NaOH(aq) CH 3 COONa(aq) + H 2 O(1) HNO 3 (1) + H 2 SO 4 (1) H 2 NO 3+ (aq) + HSO 4 - (aq) (iii) NaHSO 4 (aq) + NaOH(aq) Na 2 SO 4 (aq) + H 2 O (l) [3] c) When dissolved in water, ethylamine, CH 3 NH 2, reacts as follows: CH 3 NH 2 (aq) + H 2 O(1) ¾ CH 3 NH 3+ (aq) + OH - (aq) (i) Explain why ethylamine is regarded as a BrØnsted-Lowry base. Identify the species in the above equation which is the conjugate acid. d) Write equations to show how the hydrogensulfate ion, HSO 4 -, can act as both an acid and a base in aqueous solution. e) Concentrated phosphoric acid reacts with bromide ions as follows: H 3 PO 4 (aq) + Br - (s) ¾ HBr(g) + H 2 PO 4- (aq) Identify the base and the conjugate base in this reaction. Base... Conjugate base... [2] TOTAL / 15

8 TOPIC 19 Questionsheet 7 THEORY OF INDICATORS a) Give three features that a substance must possess if it is to be suitable as an acid-base indicator. b) Acid-base indicators are normally regarded as weak acids, which may be represented by the formula HIn (where In - is the indicator anion). (i) Write an equation for the dissociation in water of an indicator HIn, and hence derive an expression for its indicator constant, K in. [2] What is meant by the term working range of an indicator? (iii) An indicator has a pk in of 5.0; over what ph range would you expect it to change colour? c) The pk in values for three indicators are given below: methyl orange 3.7 bromothymol blue 7.0 thymolphthalein 9.7 State, with reasons, which of these you would choose for a titration of: (i) ethanoic acid (pk a = 4.76) with potassium hydroxide. hydrochloric acid with ethylamine (pk a = 10.7). (iii) ethanoic acid with ethylamine. TOTAL / 16

9 TITRATION CURVES a) The acid dissociation constant, K a, for propanoic acid is 1.35 x 10-5 mol dm -3. Sketch as accurately as possible on the axes below the ph titration curve you would expect to obtain when 20.0 cm 3 of a solution of propanoic acid, containing 0.1 mol dm -3, is titrated with a solution of sodium hydroxide of the same concentration. Show on your graph the initial ph of the acid, the volume of sodium hydroxide used at the end-point, and the ph at the half-equivalence point. 14 TOPIC 19 Questionsheet 8 ph 0 0 Volume NaOH added 40 [5] b) When 25.0 cm 3 of a weak organic acid HA were titrated with 0.2 M sodium hydroxide solution, 18.0 cm 3 of the latter were required for neutralisation. In a subsequent experiment, 9.0 cm 3 of alkali were added to 25.0 cm 3 of the acid; the ph of the resulting solution was Calculate: (i) the concentration of the acid in mol dm -3. the acid dissociation constant, K a. (iii) the ph of the original acid solution. c) On the following axes, sketch the ph titration curves you would expect to obtain for the titration of: (i) 0.1 M NaOH and 0.1 M NH 3 (aq) (pk a = 9.25) with 0.1 M HNO 3. On your sketch, indicate the region over which the ammonia-nitric acid mixture would act as a buffer. 14 ph 0 0 Volume 0.1 M HNO 3 added [5] TOTAL / 16

10 TOPIC 19 Questionsheet 9 ACID-BASE STRENGTH AND ENTHALPY OF NEUTRALISATION a) (i) Define enthalpy of neutralisation. When sulfuric acid is neutralised by sodium hydroxide solution, the enthalpy of neutralisation is -57 kj mol -1 ; the value for the neutralisation of ethanoic acid by sodium hydroxide is -54 kj mol -1. Write chemical equations for these two reactions and explain the difference between the two values. Chemical equations Explanation of difference... b) An experiment to determine the enthalpy of neutralisation of sodium hydroxide with hydrochloric acid produced a value of kj mol -l ; the temperature rise in this experiment was 10.8 K. A similar experiment performed with hydrocyanic acid and ammonia, in which the concentrations of the solutions and the volumes used were the same as in the previous experiment, produced a temperature rise of 1 K. (i) Estimate the enthalpy of neutralisation of hydrocyanic acid (HCN) by ammonia. Explain the difference between the two values obtained.... [4] (iii) Suggest the result you might expect to obtain if hydrocyanic acid were neutralised with sodium hydroxide solution (assuming similar concentrations and volumes). TOTAL / 15

11 TOPIC 19 Questionsheet 10 BUFFER SOLUTIONS a) (i) What do you understand by the term buffer solution? Give one example each of an acid buffer, an alkaline buffer, and a buffer in a biological system. An acid buffer... [1] An alkaline buffer... [1] A buffer in a biological system... [1] b) (i) What do you understand by the expression buffering capacity of a buffer solution? State two factors that will affect the buffering capacity of such a solution. c) (i) Write ionic equations to show how a buffer solution containing ethanoic acid and sodium ethanoate reacts with added H + and OH - ions. Reaction with H + ions... [1] Reaction with OH - ions... [1] The amino acid glycine, H 2 N COOH, is most effective as a buffer at ph values of about 2.3 and 9.6. Assuming that glycine exists as zwitterions rather than molecules, write down the formulae of all organic species present at ph 2.3 and write equations to show how added H + and OH - ions would be removed at this ph. Species present... [2] Reaction with H + ions... [1] Reaction with OH - ions... [1] d) By reference to your knowledge of buffer solutions, explain why, when ethanoic acid is titrated with sodium hydroxide solution, the ph at first changes sharply, and then remains almost unchanged until the end-point of the titration is approached.... [5] TOTAL / 19

12 TOPIC 19 Questionsheet 11 ph OF BUFFER SOLUTIONS a) Calculate the ph of an aqueous solution containing: (i) mol dm -3 ethanoic acid (CH 3 CO 2 H) and mol dm -3 sodium ethanoate. K a for ethanoic acid is mol dm mol dm -3 of bromoethanoic acid (Br CO 2 H) and mol dm -3 sodium bromoethanoate. K a for bromoethanoic acid is mol dm -3 (iii) mol dm -3 of bromoethanoic acid (Br CO 2 H) and mol dm -3 sodium bromoethanoate. K a for bromoethanoic acid is mol dm -3 b) Propanoic acid behaves as a weak acid: CH 3 COOH(aq) ¾ CH 3 COO - (aq) + H + (aq) (i) Write an expression for the acid dissociation constant, Ka, for propanoic acid. A buffer solution of ph 5.0 was prepared by mixing equal volumes of propanoic acid and mol dm -3 sodium propanoate, CH 3 COO-Na +. Calculate the concentration of propanoic acid used to produce this buffer solution. (pka for propanoic acid is 4.87)... [5] [1] TOTAL / 13

13 TOPIC 19 Questionsheet 12 TEST QUESTION I a) A compound was found on analysis to contain: S 26.89% O 13.45% Cl 59.66% Determine its empirical formula. b) 0.05 mol of this compound was dissolved in water and the solution made up to 1.0 dm cm 3 of this solution required 25.0 cm 3 of 0.1 M silver nitrate solution to complete the precipitation of chloride ions. Calculate the number of moles of chloride ions precipitated, and hence determine the molecular formula of the compound. c) A second 25.0 cm 3 portion of the solution was exactly neutralised by sodium hydroxide solution. This neutralised solution, on treatment with excess barium chloride solution, produced g of a white precipitate; this precipitate dissolved in acid, releasing sulfur dioxide. Identify the white precipitate. Given that the compound reacts with water to form a strong monobasic acid HP, and a weak dibasic acid HQ but no other products, identify HP and HQ and write an equation for the reaction of the compound with water.... [6] d) The ph of the solution was measured and found to be (i) Calculate the hydrogen ion concentration in the solution. How many moles of hydrogen ions are produced by the dissociation of 0.05 mol of the weak acid, HQ? (iii) Assuming that the second dissociation of the weak acid can be ignored, calculate the percentage of the total hydrogen ion concentration which is attributable to the weak acid. TOTAL / 16

14 TOPIC 19 Questionsheet 13 TEST QUESTION II In a thermometric titration, 20.0 cm 3 aliquots of a weak acid HZ were titrated individually with 1.0 M sodium hydroxide solution; the maximum temperature rise was recorded for each experiment. The results are given below. Volume 1.0 M NaOH used /cm 3 Maximum temperature rise / C a) Plot these results on the graph paper printed below. [5] TOTAL (Continued...) /

15 TOPIC 19 Questionsheet 13 Continued TEST QUESTION II b) Calculate the concentration of the acid in mol dm -3. c) The ph of the original acid was Calculate K a. d) Determine the enthalpy of neutralisation of this acid by sodium hydroxide. Assume that all solutions have a density of 1.0 g cm -3 and a specific heat capacity of 4.2 J K -1 g -1. TOTAL / 14