Solid ${\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{O}}_4$ is slowly added to a solution which is $0.020\mathrm{ }\mathrm{M}$ in $\mathrm{B}\mathrm{a}{\left({\mathrm{N}\mathrm{O}}_3\right)}_2$ and $0.020\mathrm{ }\mathrm{M}$ in $\mathrm{P}\mathrm{b}{\left({\mathrm{N}\mathrm{O}}_3\right)}_2$. Assume that there is no increase in volume on adding ${\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{O}}_4$ . There preferential precipitation takes place. What is the concentration of ${\text{Ba}}^{\text{2+}}$ when ${\text{PbSO}}_{\text{4}}$ starts to precipitate?

${\left[\mathrm{K}\right.}_{\mathrm{s}\mathrm{p}}\left({\mathrm{B}\mathrm{a}\mathrm{S}\mathrm{O}}_4\right)=1.0\mathrm{ }\times \mathrm{ }{10}^{-10}$ and ${\mathrm{K}}_{\mathrm{s}\mathrm{p}}\left({\mathrm{P}\mathrm{b}\mathrm{S}\mathrm{O}}_4\right)=1.6\mathrm{ }\times \mathrm{ }\left.{10}^{-8}\right]$

• A

    5.0 × 10^-9   M
• B

    8.0 × 10^-7  M
• Correct answer
  C

   1.25 × 10^-4M
• D

     1.95 × 10^-8M

Hence, the correct answer is option: 'C'.