WAEC Chemistry 2011 Paper

Concept:Acid strength in hydrohalic acids depends on the $\mathrm{H-X}$ bond strength. A weaker bond makes the acid stronger.Explanation:The hydrohalic acids are $\mathrm{HF}$, $\mathrm{HCl}$, $\mathrm{HBr}$, and $\mathrm{HI}$.Down Group 17, atomic size increases and bond length increases.So the $\mathrm{H-X}$ bond strength decreases from $\mathrm{HF}$ to $\mathrm{HI}$.Fluorine is small and highly electronegative, so the $\mathrm{H-F}$ bond is the strongest.Thus, $\mathrm{HF}$ releases $\mathrm{H^+}$ least readily.Therefore, $\mathrm{HF}$ is the weakest hydrohalic acid.Answer:C. $\mathrm{HF}$