A Level Official Exam Material
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Unit 5: Transition Metals and
Organic Nitrogen Chemistry
Topic 14: Acid-base Equilibria
For A Level students only
Q1. A solution of sodium hydroxide of concentration 0.0080 mol dm–3 has a pH A
between 7 and 9
B between 9 and 11
C between 11 and 13
D above 13
(Total for question = 1 mark)
Q2. A solution of sodium hydroxide of concentration 0.0080 mol dm–3 has a pH
A between 7 and 9
B between 9 and 11
C between 11 and 13
D above 13
(Total for question = 1 mark)
Q3. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
What is the pH of 0.010 mol dm−3 aqueous calcium hydroxide, Ca(OH)2(aq)?
[pKw = 14]
A 11.7
B 12.0
C 12.3
D 13.3
(Total for question = 1 mark)
Q4. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
At 25 °C, the pH of pure water is 7.00 and at 100 °C, the pH of pure water is 6.14.
What can be deduced from this information?
Q5. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Propanone reacts with iodine in both acidic and alkaline conditions.
(a) The products of the reactions under the stated conditions include (1)
(b) The rate equation for the reaction between propanone and iodine in acidic conditions is rate = k[H+ ][CH3COCH3]
The reaction was carried out at two different pH values, all other conditions remaining unchanged.
In the first reaction pH = 2.0 In the second reaction the rate was found to be 1 ⁄3 of the original value.
What was the pH of the second reaction, to 1 decimal place? (1)
A 0.7
B 1.5
C 2.5
D 2.7
(Total for question = 2 marks)
Q6. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
This question is about weak acids. pKa of ethanoic acid, CH3COOH = 4.8 pKa of chloroethanoic acid, CH2ClCOOH = 2.9
(a) What is the pH of a 0.100 mol dm−3 solution of chloroethanoic acid? (1)
A 0.27
B 1.95
C 2.90
D 3.90
(b) Which is the acid-conjugate base pair in the reaction between ethanoic acid and chloroethanoic acid? (1)
Q7. Propanal reacts very slowly with HCN at 298 K.
To increase the rate of reaction potassium cyanide, KCN, is added.
(ii) Explain why the reaction between propanal and HCN in the absence of KCN is very slow, referring to the value of Ka. No calculation is required.
[For HCN, Ka = 4.9 × 10–10 mol dm–3 ] (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 2 marks)
Q8. This question is about compounds with the molecular formula C6H12O2.
(a) Hexanoic acid, C5H11COOH, is a weak acid.
(i) Write the equation for the acid dissociation constant, Ka, of hexanoic acid. (1)
(ii) Calculate the pH of a 0.100 mol dm–3 solution of hexanoic acid.
[pKa of hexanoic acid = 4.88] (4)
(b)(i) Compound A is thought to be another isomer of hexanoic acid.
10 g of compound A is found to contain 6.21 g of carbon and 1.03 g of hydrogen, with the remainder being oxygen.
Use the data to calculate the empirical formula of compound A.
You must show all your working. (3)
(Total for question = 8 marks)
Q9. Which indicator should be used to determine the end point in a titration of a strong acid with a weak base? (1)
A universal indicator
B methyl orange
C phenolphthalein
D litmus
(Total for question = 1 mark)
Q10. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A diprotic acid, H2A, was titrated with sodium hydroxide solution.
H2A(aq) + 2NaOH(aq) → Na2A(aq) + 2H2O(l)
A 25.0 cm3 portion of 0.100 mol dm−3 sodium hydroxide solution required 12.80 cm3 of the solution of the diprotic acid for complete neutralisation.
What is the concentration of H2A in mol dm−3?
A 2.56 × 10−2
B 9.77 × 10−2
C 1.95 × 10−1
D 3.91 × 10−1
(Total for question = 1 mark)
Q11. Answer the questions with a cross in the boxes you think are correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The graphs show the changes in pH during a series of titrations.
Each titration involves two solutions, each of concentration 0.1 mol dm−3 .
(a) Which graph has ethanoic acid as one of the reactants? (1)
A Graph 1
B Graph 2
C Graph 3
D Graph 4
(b) Which graph shows a solution that has an acidic pH at the end-point? (1)
A Graph 1
B Graph 2
C Graph 3
D Graph 4
(c) Which indicator is suitable for the titration shown in Graph 3?
Use your Data Booklet. (1)
A bromocresol green
B bromothymol blue
C phenol red
D thymol blue (acid)
Use this space for any rough working.
Anything you write in this space will gain no credit.
(Total for question = 3 marks)
Q12. Which two solutions, both of concentration 0.10 mol dm–3 , have been used to produce this titration curve?
Q13. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
An example of a diprotic acid is cis-butenedioic acid.
Titration of this acid using sodium hydroxide solution gave the titration curve shown.
Which indicator would be most suitable for measuring the end-point of the neutralisation of the proton in cis-butenedioic acid which has a pKa = 6.33?
Use your Data Booklet.
A bromocresol green
B bromothymol blue
C litmus
D phenolphthalein
(Total for question = 1 mark)
Q14. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A titration was carried out by adding 0.1 mol dm−3 hydrochloric acid to 0.1 mol dm−3 aqueous ammonia.
HCl(aq) + NH3(aq) → NH4Cl(aq)
The titration curve is shown.
(a) Which region of the graph represents the most effective buffer solution? (1)
A region T
B region U
C region V
D region W
(b) Which of these is the best indicator to use in this titration?
[Refer to the Data Booklet] (1)
A methyl red
B phenol red
C phenolphthalein
D thymol blue
(c) What is the approximate pH of an ammonium chloride solution? (1)
A 2.0
B 5.8
C 9.7
D 11.3
(Total for question = 3 marks)
Q15. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A graph of pH against volume of acid added for an acid-base titration is shown.
(a) Which acidic solution was used in the titration? (1)
A 0.1 mol dm–3 CH3COOH B 1.0 mol dm–3 CH3COOH C 0.1 mol dm–3 HCl D 1.0 mol dm–3 HCl
(b) Which basic solution was used in the titration? (1)
A NH3
B LiOH
C Ba(OH)2
D NaOH
(c) A student suggested five indicators that might be used in this titration: thymol blue methyl orange bromophenol blue bromocresol green phenolphthalein
How many of these indicators would be suitable?
Use your Data Booklet. (1)
A 5
B 4
C 3
D 2
(Total for question = 3 marks)
Q16. This question is about acids. (a) Calculate the pH of the following acidic solutions.
Give your answers to two decimal places.
(i) 0.14 mol dm–3 solution of hydrochloric acid (1)
(ii) 0.14 mol dm–3 solution of ethanoic acid (Ka = 1.76 × 10–5 mol dm–3 ) (3)
(b) The graph shows the titration curve for a weak acid with a strong base.
The equivalence point is A and the volume of alkali added at the equivalence point is C.
Volume B is half of volume C.
(i) Use the graph to determine the dissociation constant, Ka, of the weak acid. (3)
(ii) Explain the rapid rise in pH from 2.8 to 4 at the start of the titration. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) In another experiment, 10 cm3 of 1.0 mol dm–3 sodium hydroxide was added to 30 cm3 1.0 mol dm–3 propanoic acid (Ka = 1.3 × 10–5 mol dm–3 ).
(i) Calculate the pH of the resulting solution. (3)
(ii) State one assumption you have made in this calculation. (1) ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 13 marks)
Q17. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A buffer solution contains ethanoic acid, with a concentration of 0.10 mol dm−3 , and sodium ethanoate, with a concentration of 0.050 mol dm−3 .
Ka for ethanoic acid = 1.7 × 10−5 mol dm−3
The pH of this buffer solution is
A 2.88
B 4.47
C 4.77
D 5.07
Use this space for any rough working. Anything you write in this space will gain no credit.
(Total for question = 1 mark)
Q18. Sodium hydrogensulfate is a widely used acid, with applications that include removing limescale and as a food additive.
Sodium hydrogensulfate is a weak acid because of the presence of the hydrogensulfate ion, .
(a)(i) Write the equation for the dissociation of the hydrogensulfate ion in aqueous solution.
State symbols are not required. (1)
(ii) A solution of sodium hydrogensulfate has pH = 1.13
Calculate the concentration of this solution, in g dm−3 . (5)
(iii) State the assumptions you have used in (a)(ii). (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) A solution containing sodium hydrogensulfate and sodium sulfate is a buffer that is used to preserve urine for steroid analysis.
(i) State what is meant by the term buffer. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) Calculate the pH of the buffer prepared by dissolving 0.750 mol of sodium hydrogensulfate and 0.500 mol of sodium sulfate in distilled water to make 1.00 dm3 of solution. (3)
(iii) Separate samples of 0.00500 mol of hydrochloric acid are added to 1.00 dm3 of distilled water and to the buffer in (b)(ii).
Calculate the pH changes that result in each case.
Assume that the volumes remain constant at 1.00 dm3 . (4)
(c) The titration curve obtained from the addition of sodium hydroxide solution to a weak acid is shown.
The equivalence point (E) of this titration occurred at pH = 8
Explain the observations that would be made if methyl orange (pKIn = 3.7) were used as the indicator for this titration. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 20 marks)
Q19. The equation for the formation of ammonia in the Haber Process is shown
(e) Ammonia from the Haber Process reacts with acids.
With phosphoric acid, H3PO4, a number of products are formed in solution.
One of these is the fertiliser diammonium hydrogenphosphate.
(i) Write an equation for the production of this fertiliser.
State symbols are not required. (2)
(ii) Write an ionic equation to show that ammonium ions are acidic in aqueous solution.
State symbols are not required. (1)
(iii) A solution containing both ammonia and ammonium ions acts as a buffer.
Explain, using a relevant ionic equation, the effect of adding a small amount of acid to this buffer. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 6 marks)
Q20. This question is about sulfuric acid and its salts.
(b) Sulfur trioxide is used to produce sulfuric acid.
(i) Commercial concentrated sulfuric acid contains 98.5 % H2SO4 and 1.5 % water by mass.
The density of concentrated sulfuric acid is 1800 g dm−3 .
Calculate the concentration of this sulfuric acid in mol dm−3 . (2)
(ii) The pH of a 0.10 mol dm−3 solution of sulfuric acid at 25 °C is 0.97.
Calculate the concentration of hydrogen ions, in mol dm−3 , in this solution. (1)
(iii) In an aqueous solution of sulfuric acid, the following equilibria exist.
Explain, in terms of these equilibria, why the concentration of hydrogen ions in a 0.10 mol dm−3 solution of sulfuric acid is not 0.20 mol dm−3 .
No calculation is required. (2)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
Q21. The compound lactic acid can be synthesised from ethanal in two steps.
(b) Sodium hydrogencarbonate, NaHCO3 , has been used by some athletes to help prevent lactic acid causing muscle pain during exercise. Write an equation for the reaction between sodium hydrogencarbonate and lactic acid. (1)
(c) Sodium hydrogencarbonate is part of a buffer in the body that controls the pH of blood. Two of the equilibria involved in this process are shown.
(i) Use the equilibria to explain how the buffer keeps the pH of blood nearly constant when a small increase in the concentration of hydrogen ions occurs. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) The pH of a blood sample was found to be 7.41.
Calculate the ratio of the concentration of in the blood sample.
(Total for question = 18 marks)
Q25. This question concerns acidic solutions.
(a) (i) A solution of hydrochloric acid has a pH of 1.125.
Calculate the concentration of hydrogen ions and of hydroxide ions, in mol dm−3 , in this solution.
[Kw = 1.00 × 10−14 mol2 dm−6 ] (2)
(ii) Calculate the volume of water which must be added to 25.0 cm3 of this solution to increase the pH from 1.125 to 1.500, giving your answer to an appropriate number of significant figures. (4)
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page