Titration

2 marks · Hard

During a titration, a student adds hydrochloric acid to sodium hydroxide solution from a burette. The student does a 'rough titration' first, then repeats the procedure more carefully. Explain the purpose of doing a rough titration, and describe one way to increase accuracy when repeating the titration.

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Also accept: Rough titration gives an approximate volume needed for neutralisation; when repeating, add most of the acid quickly up to just before endpoint, then add the acid dropwise for accuracy; The rough titration shows how much is needed, then in repeats you add quickly to near endpoint then dropwise

Common mistakes: Saying rough titration is for making mistakes/for practice, rather than to estimate endpoint; Not describing dropwise addition to increase accuracy

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Marking points

B1 · 1 mark · Explains the purpose of a rough titration (to obtain approximate volume needed for neutralisation or to estimate the endpoint)

B2 · 1 mark · Describes one way to improve accuracy in repeat (e.g. add acid quickly up to near endpoint, then dropwise, or swirl flask throughout, or read burette more precisely)

Worked solution

A rough titration is done first to find out approximately how much acid is needed to neutralise the alkali. When repeating, you increase accuracy by running most of the acid in quickly, then adding it dropwise as you approach the endpoint (or swirl the flask to mix, or read burette precisely to 2 d.p.).