${K_a} = 7.2 \times {10^{ - 10}},\,\,v = 100\,,\,\,\,{x^2} = \frac{{{K_w}}}{{{K_a}}} \times v$
${(0.037)^2} = \frac{{{K_w} \times 100}}{{7.2 \times {{10}^{ - 10}}}}$
${K_w} \times 100 = {(0.037)^2} \times 7.2 \times {10^{ - 10}}\,$
$ \Rightarrow \,\,\,{K_w} = \frac{{0.037 \times 0.037 \times 7.2 \times {{10}^{ - 10}}}}{{100}} = 0.986 \times {10^{ - 14}}$
($PbCl_2$ નો $K_{SP}$ $ = 3.2 \times 10^{-8}$; $Pb$ નું પરમાણ્વીય દળ $= 207\, u$)