RH₂ is an ion exchange resin used to purity water in RO.
It can replace Ca²⁺ in hard water.

${\mathrm{RH}}_2+{\mathrm{Ca}}^{2+}⟶\mathrm{RCa}+2{\mathrm{H}}^{+}$

Water coming out of ion exchange resin has [H⁺] = 0.01 M. Thus, hardness of water in ppm Ca²⁺ ion is

$$\begin{gathered} {\mathrm{RH}}_2+{\mathrm{Ca}}^{2+}\to \mathrm{RCa}+2{\mathrm{H}}^{+} \\ 1 \mathrm{mol} 2 \mathrm{mol} \\ ? 0.01 \mathrm{mol} {\mathrm{L}}^{-1} \\ \text{ When }\left[{\mathrm{H}}^{+}\right]\text{is }2\mathrm{mol}\text{ then, }\left[{\mathrm{Ca}}^{2+}\right]\text{ is }=1\mathrm{mol} \\ \text{ When }\left[{\mathrm{H}}^{+}\right]\text{is }0.01\mathrm{mol}\text{ then }\left[{\mathrm{Ca}}^{2+}\right]\text{ is }=\frac{0.01}{2}{\mathrm{molL}}^{-1} \\ \begin{array}{l}\therefore \left[{\mathrm{Ca}}^{2+}\right]=0.005\mathrm{M}=0.005\times 40g{L}^{-1}\\ =0.2{\mathrm{gL}}^{-1}\\ 1000\mathrm{mL}\text{ of water has }\left[{\mathrm{Ca}}^{2+}\right]=0.2\mathrm{g}\\ {10}^6\mathrm{mL}\text{ (parts per million) has }\left[{\mathrm{Ca}}^{2+}\right]=\frac{0.2\times {10}^6}{{10}^3}=200\end{array} \end{gathered}$$