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# Given $\Delta H^\circ _{ioniz}$ (HCN) = 45.2 kJ $mol^{-1}$ and $\Delta H^\circ _{ioniz} (CH_3COOH)$ = 2.1 kJ $mol^{-1}$. Which one of the following fact is true?

(a) $pK_a (HCN) = pK_a (CH_3COOH)$
(b) $pK_a (HCN) > pK_a (CH_3COOH)$
(c) $pK_a (HCN) < pK_a (CH_3COOH)$
(d) $pK_a (HCN) = \frac{45.50}{2.1} pK_a (CH_3COOH)$

Toolbox:
• Smaller the $pK_a$ value, stronger the acid.
Answer: $pK_a (HCN) > pK_a (CH_3COOH)$
HCN has greater heat of ionisation, and is a weak acid. Thus, it has high $pK_a$ value.