Question

$N{H}_{4}CN$ is a salt of weak acid HCN $(K_a=6.2\times {10}^{-10})$ and a weak base $N{H}_{4}OH(K_b=1.8\times {10}^{-5})$. A one molar solution of $N{H}_{4}CN$ will be:

• A. neutral
• B. strongly acidic
• C. strongly basic
• D. weakly basic

Answer 1

Answer: (D)

weakly basic

Smaller the $pKa$ , the stronger the acid. Very strong acids have $pKa$ less than 1, moderately strong acids have $pKa$ in between 1 to 5 and weak acids have $pKa$ above 5.

Ka is directly proportional to the $H^{+}$ concentration and Kb is directly proportional to the $O{H}^{-}$ concentration.

In this case since $K_b>K_a$ and $K_b<<1,N{H}_{4}CN$ is weakly basic.