A student is given dilute ethanoic acid and 25.0 cm³ portions of aqueous sodium hydroxide. Fig. 6.1 shows the apparatus available. The student wants to find...

Labelled answer diagram:

A titration measures the exact volume of one solution needed to react completely with a fixed volume of another. The alkali is measured accurately into a flask with an indicator, and acid is run in from a burette until the indicator just changes colour, marking the end-point.

(a) A full method scores any four [4]:

1. Use a pipette to measure exactly 25.0 cm³ of sodium hydroxide into the conical flask [1].
2. Add a few drops of indicator to the flask [1].
3. Fill the burette with the ethanoic acid and take the initial reading at the bottom of the meniscus at eye level [1].
4. Add acid from the burette while swirling the flask, adding it drop by drop near the end-point [1].
5. Stop when the indicator just changes colour, record the final reading, and calculate titre = final − initial reading [1].

(b) A suitable indicator is phenolphthalein [1] (thymolphthalein is also accepted); with the weak acid ethanoic acid and the strong base sodium hydroxide the end-point lies on the alkaline side, where phenolphthalein changes sharply.

(c) One error that makes the measured volume too large is overshooting the end-point (adding acid past the colour change) [1] (accept: an air bubble in the burette jet that fills during the titration, or failing to swirl so extra acid is added before the colour changes).