Question

Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the following conjugate bases? $O{H}^{-},R{O}^{-},C{H}_{3}CO{O}^{-},C{l}^{-}$

Answer 1

A strong acid donates proton to water to form conjugate base and hydronium ions. As strong acids are very reactive, they should produce stable products on reaction.

So, the conjugate base formed would be very weak. That means, strong acids have very weak conjugate base or conjugate acid of a weak base are always stronger.

Conjugate pairs differ by a $H^{+}$

To find a conjugate acid of given ions we need to add $H^{+}$

$O{H}^{-}+H^{+}\to H_{2}O$

$R{O}^{-}+H^{+}\to ROH$

$C{H}_{3}CO{O}^{-}+H^{+}\to C{H}_{3}COOH$

$C{l}^{-}+H^{+}\to HCl$

$HCl,HCl{O}_4,HI,HBr,HN{O}_3,H_{2}S{O}_4$ are all inorganic acids and are strong, hence their conjugate base is much weaker than that of $H_{2}O$.

While $C{H}_{3}COOH$ is a weak acid so its conjugate base is strong. Alcohol $(R-OH)$ itself behave more like a base so after removing one $H^{+}$ its conjugate base becomes very strong conjugate base.

Therefore, the decreasing order of basicity of the given conjugate base is

$R{O}^{-}>C{H}_{3}CO{O}^{-}>O{H}^{-}>C{l}^{-}$