An acidic aqueous solution contains $0.02 \mathrm{M} - {\mathrm{FeCl}}_2$ and $0.05 \mathrm{M} - {\mathrm{FeCl}}_3$ , at 25° C. Now NaOH solution is added is this solution, drop by drop. The pH range in which only the precipitation of Fe(OH) is occur without precipitation of $\mathrm{Fe}{\left(\mathrm{OH}\right)}_2$ (s) differs by 'x' unit. The value of 'x' is given:

${\mathrm{K}}_{\mathrm{sp}}$ of $\mathrm{Fe}(\mathrm{OH}{)}_2=8\times {10}^{-16}$

${\mathrm{K}}_{\mathrm{sp}}$ of $\mathrm{Fe}(\mathrm{OH}{)}_3=4\times {10}^{-28}$