Chemistry · May / June 2024 · Paper 12 · Question 39

Two titrations of dilute hydrochloric acid with aqueous sodium hydroxide are done using two different indicators, methyl orange and thymolphthalein. The equation for the reaction is shown. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Using a volumetric pipette, 25.0 cm³ dilute hydrochloric acid is added to a conical flask with a few drops of indicator. Aqueous sodium hydroxide is added from a burette to the dilute hydrochloric acid until the end-point is reached. The end-point of the titration with each indicator occurs when 23.6 cm³ of aqueous sodium hydroxide is added. Which statements about this experiment are correct? 1 The methyl orange changes colour from red to yellow. 2 The thymolphthalein changes colour from blue to colourless. 3 The dilute hydrochloric acid is less concentrated than the aqueous sodium hydroxide.

1. A1, 2 and 3
2. B1 and 2 only
3. C1 and 3 only
4. D2 and 3 only

As printed in the paper