Question

Which of the following is/are true regarding the aqueous dissociation of $HCN$, $K_a=4.9\times {10}^{-10}$, at ${25}^o$C?
I. At equilibrium, $\left[H^{+}\right]=\left[C{N}^{-}\right]$
II. At equilibrium, $\left[H^{+}\right]>\left[C{N}^{-}\right]$
III $HC{N}_{\left(aq\right)}$ is a strong acid

• A. I only
• B. II only
• C. I and II only
• D. I and III only
• E. I, II, and III

Answer 1

Answer: (B)

II only

Only statement II is true.
I. and II.At equilibrium, $\left[H^{+}\right]>\left[C{N}^{-}\right]$
$\left[C{N}^{-}\right]$ is due to ionisation of HCN whereas $\left[H^{+}\right]$ is due to ionisation of HCN and $H_{2}O$ .
$K_w>K_a$
$K_w={10}^{-7}$
$K_a=4.9\times {10}^{-10}$
Thus, there is significant $\left[H^{+}\right]$ from ionisation of water.
III HCN(aq) is a very weak acid.