One litre of a sample of hard water contains 5.55 mg of CaCl₂ and 4.75 mg of MgCl₂. The total harness in terms of ppm of CaCO₃ is

$\begin{array}{l}55.5 \mathrm{g} {\mathrm{CaCl}}_2\equiv 50 \mathrm{g} {\mathrm{CaCO}}_3\\ 55.5 \mathrm{mg} {\mathrm{CaCl}}_2\equiv 50\times {10}^{-3} \mathrm{g} {\mathrm{CaCO}}_3\\ 47.5 \mathrm{g} {\mathrm{MgCl}}_2\equiv 50 \mathrm{g} {\mathrm{CaCO}}_3\\ 47.5 \mathrm{mg} {\mathrm{MgCl}}_2\equiv 50\times {10}^{-3} \mathrm{g} {\mathrm{CaCO}}_3\\ \text{ Total }{\mathrm{CaCO}}_3=(50+50)\times {10}^{-3}={10}^{-2} \mathrm{g} {\mathrm{L}}^{-1}\\ \mathrm{ppm}=\frac{{10}^{-2}}{1000\mathrm{mL}}\times {10}^6\mathrm{mL}=10\end{array}$