Question

Substance	$k_a$
Hydrogen carbonate ion (bicarbonate ion) $HC{O}_3^{-}$	$4.8\times {10}^{-11}$
Hydrogen phosphate ion $HP{O}_4^{-3}$	$3.6\times {10}^{-13}$
Water $H_{2}O$	$1.0\times {10}^{-14}$
Hydrogen sulfide ion $H{S}^{-}$	$1.0\times {10}^{-19}$

Using acid dissociation constants above, which substance is the strongest base?

• A. $C{O}_3^{2-}$
• B. $P{O}_4^{3-}$
• C. $O{H}^{-}$
• D. $S^{2-}$

Answer 1

The correct option is D $S^{2-}$

$K_a\times K_b=K_W={10}^{-14}$

A strong base is one which has higher value of $K_b$.

Since product of $K_a$ and $K_b$ is constant, higher value of $K_b$ corresponds to lower value of $K_a$.

Hence, a strong base has a lower value of $K_a$.

Also, weaker the acid, stronger is its conjugate base.

From the given data, $H{S}^{-}$ is the weakest acid having least value of $K_a$. Hence, its conjugate base $S^{2-}$ is the strongest.