Acids, Alkalis and Titrations
6 marks · NormalPotassium carbonate is a weak alkali. A student decides to find out how much sulfuric acid is needed to neutralise a sample of potassium carbonate solution.
State one observation that shows potassium carbonate solution is alkaline when tested with a suitable indicator.
Write the name or formula of the ion responsible for the alkaline nature of potassium carbonate solution.
The student prepares for titration by placing 25.0 cm³ of potassium carbonate solution into a conical flask. Describe how to add the acid during the titration and how to recognise when to stop.
Suggest a suitable indicator for this titration and explain why universal indicator is not used.
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(a) Indicator turns blue (or purple/pink if phenolphthalein is used)
(b) OH^-
(c) Add acid from the burette quickly at first, then dropwise as the endpoint approaches while swirling the flask. Stop adding when the indicator changes colour permanently.
(d) Use methyl orange or phenolphthalein; universal indicator is not suitable because it has a gradual range of colours and no sharp endpoint.
Common mistakes: choosing universal indicator; giving incorrect endpoint method; naming wrong ion
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States correct indicator colour showing alkaline solution, e.g. blue with litmus or pink with phenolphthalein
B2 (b) · 1 mark · Names hydroxide ion, OH^-, as responsible for alkalinity
M1 (c) · 1 mark · Describes initially adding acid quickly, then dropwise as the endpoint approaches
A1 (c) · 1 mark · States that you stop when the indicator changes colour permanently (endpoint)
B3 (d) · 1 mark · Suggests methyl orange or phenolphthalein as suitable indicator
B4 (d) · 1 mark · Explains universal indicator is unsuitable due to gradual colour change and unclear endpoint
Worked solution
(a) When an indicator such as litmus is used, it turns blue in alkaline potassium carbonate solution.
(b) The OH^- ion (hydroxide ion) is responsible for alkalinity.
(c) During titration, add sulfuric acid from the burette quickly at first. As you near the expected endpoint, add the acid dropwise while swirling the flask. Stop when the indicator changes colour permanently.
(d) Methyl orange or phenolphthalein is suitable, because they have a sharp colour change at the endpoint. Universal indicator should not be used as it changes colour gradually, making the endpoint unclear.