SL Topic 8
Practice Problems.
1. Define the terms acid and base according to the Brønsted-Lowry theory and state one example of a weak
acid and one example of a strong base.
(2)
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2.
Describe two different methods, one chemical and one physical, other than measuring the pH, that
could be used to distinguish between ethanoic acid and hydrochloric acid solutions of the same
concentration.
(4)
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3. Water is an important substance that is abundant on the Earth’s surface. Water dissociates
according to the following equation.
+
–
H2O(l)  H (aq) + OH (aq)
(i)
State the equilibrium constant expression for the dissociation of water.
(1)
(ii)
–
Explain why even a very acidic aqueous solution still has some OH ions present in it.
(1)
(iii)
State and explain the effect of increasing temperature on the equilibrium constant above
given that the dissociation of water is an endothermic process.
(3)
(iv)
The pH of a solution is 2. If its pH is increased to 6, deduce how the hydrogen ion
concentration changes.
(2)
SL Topic 8
Practice Problems.
ANSWERS:
1. acid is a proton/H+ donor and base is a proton/H+ acceptor;
H2CO3/CH3COOH and NaOH/KOH/Ba(OH)2;
Accept any suitable examples.
2
Chemical [2 max]
reaction with reactive metal/Mg/Zn/carbonate/hydrogen carbonate;
hydrochloric acid would react faster/more vigorously / ethanoic acid would
react slower/less vigorously;
OR
react with alkali;
temperature change will be more for hydrochloric acid / temperature change
will be less for ethanoic acid;
Physical [2 max]
conductivity;
hydrochloric acid will conduct more/higher / ethanoic acid will conduct
less/lower;
Accept other suitable examples.
2. (i)
Kc = / Kw = [H+][OH–]/Kw = [H3O+][OH–];
Do not award mark if [ ] are omitted or other brackets are used.
Expression must be consistent with Kc/Kw.
(ii)
(iii)
(iv)
4 max
1
–
[H+
] decreases but still some present (Kw / Kc constant) / [OH–]
cannot go to zero as equilibrium present / [OH–] , thus [OH–]
cannot be zero / OWTTE;
Accept equilibrium present.
1
(changing T disturbs equilibrium) forward reaction favoured / equilibrium
shifts to the right;
to use up (some of the) heat supplied;
(Kw / Kc) increases (as both [H+] and [OH–] increase);
3
pH = 2, [H+] = 0.01 mol dm–3 and pH = 6, [H+] = 10–6 mol dm–3 / [H+] = 10–pH;
[H+] decreased/changed by 10000/10–4;
Award [2] for correct final answer.
2
[7]