Question

Which of the following are true regarding the aqueous dissociation of HCN, $K_a$ = 4.9 x ${10}^{-10}$ at ${25}^{o}C$ ?
i. At equilibrium, $\left[H^{+}\right]=\left[C{N}^{-}\right]$
ii. At equilibrium, $\left[H^{+}\right]$ = [HCN]
iii. $HCN$ is a strong acid

• A. i only
• B. ii only
• C. i and ii only
• D. ii and iii only
• E. i, ii and iii

Answer 1

Answer: (A)

i only

$K_a$ of $HCN=4.9\times {10}^{-10}$ at ${25}^{o}C$ which is significantly less. Hence it is a weak acid.

$HCN⟶H^{+}+C{N}^{-}$

Initially $100$

At equilibrium $1-xxx$

Hence at equilibrium $\left[H^{+}\right]=\left[C{N}^{-}\right]$