Temporary hardness is due to bicarbonates of${\mathrm{Mg}}^{2+}$ and${\mathrm{Ca}}^{2+}$. It is removed by addition of CaO as follows:

$\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2+\mathrm{CaO}⟶2{\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$

Mass of Cao required to precipitate 2g ${\mathrm{CaCO}}_3$ is :