Determine the maximum number of moles of ${\mathrm{MgF}}_2$ that can dissolve in $1000 \mathrm{L}$ of a buffer solution of $\mathrm{pH}=4.$ Given: ${\mathrm{K}}_{\mathrm{sp}}$ of ${\mathrm{MgF}}_2=\frac{9}{14}\times {10}^{-8}$ and ${\mathrm{K}}_{\mathrm{a}}$ of $\mathrm{HF}=3.5\times {10}^{-4}.$ Take $(2.17{)}^3=10$.

$\left(\mathrm{a}\right)$ Determine the concentration of ${\mathrm{NH}}_3$ solution whose $1 \mathrm{L}$ can dissolve $0.1$ mole ${\mathrm{CuCO}}_3$.

Given $:{\mathrm{K}}_{\mathrm{sp}}$ of ${\mathrm{CuCO}}_3=1.4\times {10}^{-10}$ and ${\mathrm{K}}_{\mathrm{f}}$ ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}=2\times {10}^{13}.$ Take $\sqrt[4]{\frac{1000}{2.8}}=4.4.$ Assume no other reaction to take place.

$\left(\mathrm{b}\right)$ An aqueous solution of a metal bromide, $\mathrm{MBr}{}_2(0.05 \mathrm{M})$ is saturated with ${\mathrm{H}}_2\mathrm{S}.$ Calculate the minimum $\mathrm{pH}$ at which the metal sulphide will be precipitated. Concentration of ${\mathrm{H}}_2\mathrm{S}$ in a saturated solution$=0.1 \mathrm{M}$ $\left[{\mathrm{K}}_{\mathrm{a}1}\right.$ of $\left({\mathrm{H}}_2\mathrm{S}\right)={10}^{-7}, {\mathrm{K}}_{\mathrm{a}2}$ of $\left({\mathrm{H}}_2\mathrm{S}\right)=1.2\times {10}^{-13},{\mathrm{K}}_{\mathrm{sp}}$ for$\left.\mathrm{MS}=6\times {10}^{-21}\right]$.