A LEVEL CHEMISTRY
TOPIC 12 – ACIDS, BASES AND BUFFERS
TEST
Answer all questions
Max 50 marks

Name		……………………………………………………………..

Mark		……../50	……....%	Grade ………

1.	
Water dissociates slightly according to the equation:
H2O(I)      H+(aq)   +   OH−(aq)
The ionic product of water, Kw, is given by the expression
Kw = [H+][OH−]
Kw varies with temperature as shown in the table.
 
 	Temperature / °C	Kw / mol2 dm−6	
 	25	1.00 × 10−14	
 	50	5.48 × 10−14	

     (a)     Explain why the expression for Kw does not include the concentration of water.
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(2)
(b)     Explain why the value of Kw increases as the temperature increases.
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(2)
(c)     Calculate the pH of pure water at 50 °C.
Give your answer to 2 decimal places.
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(3)
(d)     Calculate the pH of 0.12 mol dm−3 aqueous NaOH at 50 °C.
Give your answer to 2 decimal places.
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(3)
(Total 10 marks)

2.      (a)     A sample of hydrochloric acid has a pH of 2.34
Write an expression for pH and calculate the concentration of this acid.
pH ................................................................................................................
Concentration ..............................................................................................
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(2)
(b)     A 0.150 mol dm–3 solution of a weak acid, HX, also has a pH of 2.34
(i)      Write an expression for the acid dissociation constant, Ka, for the acid HX.
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(ii)     Calculate the value of Ka for this acid and state its units.
Calculation .........................................................................................
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Units ....................................................................................................
(iii)     Calculate the value of pKa for the acid HX. Give your answer to two decimal places.
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(5)

(c)     A 30.0 cm3 sample of a 0.480 mol dm–3 solution of potassium hydroxide was partially neutralised by the addition of 18.0 cm3 of a 0.350 mol dm–3 solution of sulphuric acid.
(i)      Calculate the initial number of moles of potassium hydroxide.
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(ii)     Calculate the number of moles of sulphuric acid added.
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(iii)    Calculate the number of moles of potassium hydroxide remaining in excess in the solution formed.
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(iv)    Calculate the concentration of hydroxide ions in the solution formed.
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(v)     Hence calculate the pH of the solution formed. Give your answer to two decimal places.
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(6)
(Total 13 marks)

3.      The value of the acid dissociation constant, Ka, for the weak acid HA, at 298 K, is
1.45 × 10–4 mol dm–3.
(a)     Write an expression for the term Ka for the weak acid HA.
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(1)
(b)     Calculate the pH of a 0.250 mol dm–3 solution of HA at 298 K.
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(4)
(c)     A mixture of the acid HA and the sodium salt of this acid, NaA, can be used to prepare a buffer solution.
(i)      State and explain the effect on the pH of this buffer solution when a small amount of hydrochloric acid is added.
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(ii)     The concentration of HA in a buffer solution is 0.250 mol dm–3. Calculate the concentration of A– in this buffer solution when the pH is 3.59
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(6)
(Total 11 marks)

 

4.      (a)     	Titration curves labelled A, B, C and D for combinations of different acids and bases 		are shown below. All solutions have a concentration of 0.1 mol dm–3.

(i)      Select from A, B, C and D the curve produced by the addition of
ammonia to 25 cm3 of hydrochloric acid  ................................................
ethanoic acid to 25 cm3 of sodium hydroxide  .........................................
sodium hydroxide to 25 cm3 of hydrochloric acid  ...................................
(ii)     A table of acid–base indicators and the pH ranges over which they change colour is shown below.
 
 	Indicator	pH range	 	
 	Thymol blue	1.2 – 2.8	 	
 	Bromophenol blue	3.0 – 4.6	 	
 	Methyl red	4.2 – 6.3	 	
 	Cresolphthalein	8.2 – 9.8 	 	
 	Thymolphthalein	9.3 – 10.5 	 	

Select from the table an indicator which could be used in the titration which produces curve A but not in the titration which produces curve B.
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(4)
(b)     (i)      Write an expression for the term pH.
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(ii)     A solution of potassium hydroxide has a pH of 11.90 at 25°C. Calculate the concentration of potassium hydroxide in the solution.
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(4)

(c)     The acid dissociation constant, Ka, for propanoic acid has the value of 
1.35 × 10–5mol dm–3 at 25 °C.

In each of the calculations below, give your answer to 2 decimal places.
(i)      Calculate the pH of a 0.117 mol dm–3 aqueous solution of propanoic acid.
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(ii)     Calculate the pH of a mixture formed by adding 25 cm3 of a 0.117 mol dm–3 aqueous solution of sodium propanoate to 25 cm3 of a 0.117 mol dm–3 aqueous solution of propanoic acid.
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(5)
(Total 13 marks)

6.	The pH of 0.001 M NaOH at 25°C is
A       13
B       11
C       9
D       3
(Total 1 mark)

7.	A weak acid HA dissociates in aqueous solution as shown below
  HA(aq) ⇌ H+(aq) + A−(aq)                         ∆H = +20 kJ mol−1
Which one of the following changes will result in a decrease in the pH of an aqueous solution of the acid?
A       addition of a little aqueous sodium hydroxide solution
B       raising the temperature of the solution
C       dissolving a little of the sodium salt, NaA, in the solution
D       adding a platinum catalyst to the solution
(Total 1 mark)

8.	This question is based on the reactions and compounds shown in the scheme below.
 
A 0.100 mol dm−3 solution of X is found to have a pH of 2.50. The value of Ka in mol dm−3 is
A       3.16 × 10−2
B       3.16 × 10−3
C       1.00 × 10−4
D       1.00 × 10−5
(Total 1 mark)