WAEC Chemistry 2021 Paper

Concept:The titration curve for a strong acid with a strong base is S-shaped, starting at low $\mathrm{pH}$ and ending at high $\mathrm{pH}$.Explanation:Initially, the strong acid solution has a low $\mathrm{pH}$ due to a high concentration of $\mathrm{H^+}$ ions.As the strong base is added gradually, the $\mathrm{pH}$ increases slowly.Near the equivalence point, a very small volume of base causes a sharp rise in $\mathrm{pH}$.For a strong acid–strong base titration, this steep rise is centred around $\mathrm{pH} \approx 7$.After excess base is added, the $\mathrm{pH}$ becomes high and remains nearly constant.Hence, the correct graph is the one that shows a low starting $\mathrm{pH}$, a steep vertical rise around $\mathrm{pH}$ 7, and a high final $\mathrm{pH}$.Answer:Graph II (Option B)