Chemistry - Ionic Equilibrium Question with Solution | TestHub

$${\mathrm{Ag}}^{+}+{\mathrm{Br}}^{-}\rightleftharpoons \mathrm{AgBr}$$ Precipitation starts when ionic product just exceeds solubility product $$\begin{array}{rl}& {\mathrm{K}}_{\text{sp }}=\left[{\mathrm{Ag}}^{+}\right]\left[{\mathrm{Br}}^{-}\right]\\ & \left[{\mathrm{Br}}^{-}\right]=\frac{{\mathrm{K}}_{\mathrm{sp}}}{\left[{\mathrm{Ag}}^{+}\right]}=\frac{5\times 10^{-13}}{0.05}=10^{-11}\end{array}$$ i.e., precipitation just starts when $10^{-11}$ moles of $\mathrm{KBr}$ is added to $1\mathrm{L}$ of ${\mathrm{AgNO}}_3$ solution. No. of moles of $\mathrm{KBr}$ to be added $$\begin{array}{rl}\therefore \text{ weight of }\mathrm{KBr}\text{ to be added }& =10^{-11}\times 120\\ & =1.2\times 10^{-9}\mathrm{g}\end{array}$$