When hydrochloric acid is added to cobalt nitrate solution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink. On the basis of this information mark the correct answer.

$$\mathrm{\mathop {{{[Co({H_2}{O_6})]}^{3 + }}}\limits_{(Pink)} (aq) + 4C{l^ - }(aq)}\rightleftharpoons \mathrm{\mathop {{{[CoC{l_4}]}^{2 - }}}\limits_{(Blue)} (aq) + 6{H_2}O}(l)$$

The pH of neutral water at $25^{\circ} \mathrm{C}$ is 7.0 . As the temperature increases, ionisation of water increases, however, the concentration of $\mathrm{H}^{+}$ions and $\mathrm{OH}^{-}$ions are equal. What will be the pH of pure water at $60^{\circ} \mathrm{C}$ ?

The ionisation constant of an acid, $K_a$ is the measure of strength of an acid. The $K_a$ values of acetic acid, hypochlorous acid and formic acid are $1.74 \times 10^{-5}, 3.0 \times 10^{-8}$ and $1.8 \times 10^{-4}$ respectively. Which of the following orders of pH of $0.1 \mathrm{~mol} \mathrm{dm}^{-3}$ solutions of these acids is correct?

$K_{a_1}, K_{a_2}$ and $K_{a_3}$ are the respective ionisation constants for the following reactions.

$$\begin{aligned} & \mathrm{H}_2 \mathrm{~S} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HS}^{-} \\ & \mathrm{HS}^{-} \rightleftharpoons \mathrm{H}^{+}+\mathrm{S}^{2-} \\ & \mathrm{H}_2 \mathrm{~S} \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{S}^{2-} \end{aligned}$$

The correct relationship between $K_{a_1}, K_{a_2}, K_{a_3}$ is

Acidity of $\mathrm{BF}_3$ can be explained on the basis of which of the following concepts?