Calcium lactate is a salt of weak acid i.e., lactic acid having general fomula Ca₃(La)₂. Aqueous solution of salt has 0.3M concentration. pOH of solution is 5.60. If 90% of the salt dissociated then what will be the value of pK_a?

                          ${\mathrm{Ca}}_3(\mathrm{La}{)}_2\rightleftharpoons 3{\mathrm{Ca}}^{+2}+2{\mathrm{La}}^{-3}$

$\mathrm{t}={\mathrm{t}}_{\infty }$             0.3 - 0.27                 3 x 0.27        2 x 0.27

La^-3 ion is a derivative of weak acid and so it will undergo anionic hydrolysis.

P^H of such a system is given by ${\mathrm{P}}^{\mathrm{H}}=\frac{1}{2}\left[{\mathrm{P}}^{{\mathrm{K}}_{\mathrm{w}}}+{\mathrm{P}}^{{\mathrm{K}}_{\mathrm{a}}}+\mathrm{logC}\right]$

                                                 $\Rightarrow 8.4=\frac{1}{2}\left[14+P^{K_a}+\log (0.54)\right]$

                                                $\Rightarrow {\mathrm{P}}^{{\mathrm{K}}_{\mathrm{a}}}=2.8-\log (0.54)$

dissociation of salt is 90% of 0.3M = 0.27