JAMB Chemistry 2024 Paper

Concept:The indicator chosen for a titration must change colour within the pH range of the equivalence point.A strong acid and a weak base react to give an acidic equivalence point.Explanation:When a strong acid neutralises a weak base, the solution of the salt formed is acidic.The equivalence point therefore lies below $\mathrm{pH}~7$.This means an indicator that changes colour in an acidic medium is needed.$\mathrm{Methyl~orange}$ changes colour between $\mathrm{pH}~3.1$ and $\mathrm{pH}~4.4$.This places its endpoint within the acidic range, so it is the correct indicator.In contrast, $\mathrm{phenolphthalein}$ changes colour between $\mathrm{pH}~8.3$ and $\mathrm{pH}~10.0$.$\mathrm{Phenolphthalein}$ is therefore used when the equivalence point is basic, such as in a titration of a strong base with a weak acid.Hence, methyl orange is the indicator suitable for a strong acid and weak base titration.Answer:Option A: $\mathrm{methyl~orange}$