A level chemistry Acids and Bases exam questions and mark scheme

ACIDS AND BASES



A Level Chemistry complete exam questions and mark scheme

,Question 1.

(a) (i) Define the term Brnsted-Lowry acid.

(ii) What is meant by the term strong when describing an acid?

(iii) Give the value of the ionic product of water, K w, measured at 298K, and state its
units.
(4)

3 –3
(b) At 298 K, 25.0 cm of a solution of a strong acid contained 1.50 × 10 mol of hydrogen
ions.

(i) Calculate the hydrogen ion concentration in this solution and hence its pH.
3
(ii) Calculate the pH of the solution formed after the addition of 50.0 cm of 0.150 M
3
NaOH to the original 25.0 cm of acid.
(8)

(c) A solution of a strong acid was found to have a pH of 0.5

(i) Calculate the hydrogen ion concentration in this solution.
3
(ii) Calculate the volume of water which must be added to 25.0 cm of this solution to
increase its pH from 0.5 to 0.7
(5)
(Total 17 marks)

Question 2.

(a) The pH of a 0.15 M solution of a weak acid, HA, is 2.82 at 300 K.

(i) Write an expression for the acid dissociation constant, Ka, of HA, and determine
the value of Ka for this acid at 300 K, stating its units.

(ii) The dissociation of HA into its ions in aqueous solution is an endothermic process.
How would its pH change if the temperature were increased? Explain your answer.
(8)

(b) Solution A contains n moles of a different weak acid, HX. The addition of some sodium
hydroxide to A neutralises one third of the HX present to produce Solution B.

(i) In terms of the amount, n, how many moles of HX are present in Solution B?

[HX]
–
(ii) Determine the ratio [X ] in Solution B.
–4 –3
(iii) Solution B has a hydrogen ion concentration of 4.2 × 10 mol dm . Use this
information and your answer to part (b)(ii) to determine the value of the acid
dissociation constant of HX.
(5)

(c) Why is methyl orange not suitable as an indicator for the titration of HX with sodium
hydroxide?
(2)

(d) Solution B can act as a buffer. Explain what this means and write an equation that shows

, how Solution B acts as a buffer if a little hydrochloric acid is added.
(3)
(Total 18 marks)




Question 3.

(a) Explain the terms acid and conjugate base according to the Brønsted-Lowry theory.
(2)

(b) For each of the following reactions, give the formula of the acid and of its conjugate base.


–
(i) NH3 +HBr  NH 4 + Br
(1)

– 
(ii) H2SO4 + HNO3  HSO 4 + H2NO 3
(1)

(c) (i) Write an equation to represent the dissociation of water.
(1)

(ii) Give the expression for the equilibrium constant, Kc, for the reaction in (c)(i) and
use this to derive the expression for the ionic product of water, Kw.
(3)

–14 2 –6
(iii) The ionic product of water is 2.92 × 10 mol dm at 313K. Calculate the pH of
water at this temperature.
(4)

(iv) Given that the pH of water is 7.00 at 298 K, state whether the dissociation of water
is endothermic or exothermic. Give a reason for your answer.
(2)
(Total 14 marks)



Question 4.

(a) Give the Brønsted–Lowry definition of a base. State the essential feature of an acid-base
reaction in aqueous solution, writing an ionic equation to illustrate your answer.
(3)

(b) Explain what is meant by the term weak when applied to acids and bases.
(1)

(c) In aqueous solution, the weak acid propanoic acid, CH 3CH2COOH(aq), produces
–
propanoate ions CH3CH2COO (aq). Write an expression for the acid dissociation
constant, Ka, of propanoic acid and state its units.
(2)

(d) (i) Explain what is meant by the term buffer solution.

(ii) Identify two components that could be used to make a buffer solution.

(iii) Give an example of the use of a buffer solution.
(4)
(Total 10 marks)

, Page 19




Question 5.
3
(a) The graph below shows how the pH changes when 0.12 M NaOH is added to 25.0 cm of
a solution of a weak monoprotic acid, HA.

14

13

12

11

10

9

8
pH
7

6

5

4

3

2

1

0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Vo lu m e of 0 .1 2 M N a O H /cm 3




(i) Use the graph to calculate the initial concentration of the acid HA.

(ii) Write an expression for the dissociation constant. Ka, of the weak acid HA.

–
(iii) Determine the volume of sodium hydroxide added when [HA] = [ A ] and use the
graph to determine the pH at this point.

(iv) Use your answers to part (a)(ii) and part (a)(iii) to determine the value of Ka for the
acid HA.
(9)

(b) A buffer solution is formed, when approximately half of the original amount of the acid
HA(aq) has been neutralised by the base NaOH(aq). Explain how this buffer solution is
able to resist change in pH when

(i) a small amount of NaOH(aq) is added,

(ii) a small amount of HCl(aq) is added.
(4)
(Total 13 marks)