Titration
5 marks · HardA student is asked to carry out an acid-alkali titration to determine the volume of sulfuric acid needed to neutralise 25.0 cm³ of sodium hydroxide solution.
Describe, in order, four steps the student should take to set up the experiment before beginning the titration.
Explain why phenolphthalein is a suitable indicator for this titration, and why universal indicator should not be used.
State how the student would recognise the endpoint of the titration.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Any four steps in sequential order, e.g., rinse pipette, measure out sodium hydroxide, transfer to flask, add indicator, fill burette with sulfuric acid, record initial burette reading
(b) — · must show Phenolphthalein gives a sharp colour change at endpoint/pH change from colourless to pink, universal indicator gives a gradual colour change with no clear endpoint
(c) A permanent pale pink colour appears in the flask (or: the solution just turns pink and stays pink after swirling) · must show Description of colour change at endpoint (phenolphthalein)
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Marking points
M1 (a) · 1 mark · Any two of: rinse pipette; measure out 25.0 cm³ NaOH; transfer to conical flask; add few drops of indicator, fill burette/with acid/rinse burette/record initial burette reading. Max 1 for steps not in correct order.
M2 (a) · 1 mark · Two further correct steps, steps in accurate order for full marks. Must not mix up acid/alkali positions. Award one mark for 2 correct (any order), two marks for 4 correct in correct order.
B1 (b) · 1 mark · Phenolphthalein gives a sharp/clear colour change at endpoint (from colourless to pink or vice versa).
B2 (b) · 1 mark · Universal indicator not suitable because it changes gradually/has no sharp endpoint or gives range of colours.
A1 (c) · 1 mark · States the solution changes from colourless to a permanent pale pink (phenolphthalein endpoint) OR equivalent correct alternative.
Worked solution
a) Four correct steps to set up a titration:
1. Rinse the pipette with sodium hydroxide solution.
2. Use the pipette to measure 25.0 cm³ of sodium hydroxide and transfer it to a conical flask.
3. Add a few drops of phenolphthalein indicator to the flask.
4. Fill the burette with sulfuric acid solution, rinse burette with acid first, and record the initial burette reading.
b) Phenolphthalein is suitable because it gives a sharp, clear colour change at the endpoint (colourless in acid, pink in alkali); this helps determine when neutralisation occurs. Universal indicator should not be used because it changes colour gradually over a wide pH range and does not give a clear endpoint.
c) At the endpoint, a permanent pale pink colour appears in the flask (solution turns from colourless to pink and stays pink after swirling).