Question

Which of the following cation is not hydrolyzed in aqueous solution?
(i)$M{g}^{2+}$ (ii)$C{a}^{2+}$ (iii)$N{a}^{+}$ (iv)$K^{+}$

• A. (i), (ii)
• B. (iii), (iv)
• C. (i), (ii), (iii), (iv)
• D. (i), (ii), (iii)

Answer 1

The correct option is C (i), (ii), (iii), (iv)

Cations are basic radicals and anions are acidic radicals. All cations or anions from strong bases or strong acids do not hydrolyse in aqueous solution. For example, $NaCl+H_{2}O$ = no reaction. This is because-

(a) In water, strong acids and strong bases are always completely dissociated and are present in the form of solvated ions only.

(b) Because of low charge density values, these ions do not have any tendency to form covalent bond with H+or OH- of water molecules when dissolved in water. The high dielectric constant of water is also responsible to keep them apart.

(c) Hydrolysis constant of any cation is $K_h=\frac{K_w}{K_b}$. their metal hydroxides are strong bases, so their Kb values are much higher and therefore very less Kh and hence almost “no” hydrolysis.

Also the Keq for the hydrolysis of a hydrated cation is analogous to the Ka for the ionization of a weak acid. Keq is an acid ionization constant. Greater $p_{K}a$ means less ka and hence very less feasibility of hydrolysis.

So neither of the given cations undergo hydrolysis.