In the lime soda process once used in large scale municipal water softening,
calcium hydroxide prepared from lime and sodium carbonate are added to
precipitate \(\mathrm{Ca}^{2+}\) as \(\mathrm{CaCO}_{3}(s)\) and
\(\mathrm{Mg}^{2+}\) as \(\mathrm{Mg}(\mathrm{OH})_{2}(s);\)
$$
\begin{aligned}
\mathrm{Ca}^{2+}(a q)+\mathrm{CO}_{3}^{2-}(a q) & \longrightarrow
\mathrm{CaCO}_{3}(s) \\
\mathrm{Mg}^{2+}(a q)+2 \mathrm{OH}^{-}(a q) & \longrightarrow
\mathrm{Mg}(\mathrm{OH})_{2}(s)
\end{aligned}
$$
How many moles of \(\mathrm{Ca}(\mathrm{OH})_{2}\) and $\mathrm{Na}_{2}
\mathrm{CO}_{3}\( should be added to soften (remove the \)\mathrm{Ca}^{2+}$ and
\(\mathrm{Mg}^{2+}\) ) 1000 L of water in which
$$
\begin{array}{l}
{\left[\mathrm{Ca}^{2+}\right]=3.5 \times 10^{-4} \mathrm{M}} \\
{\left[\mathrm{Mg}^{2+}\right]=7.5 \times 10^{-4} \mathrm{M}}
\end{array}
$$