1 L of pond water contains 20 mg ofCa²⁺ and 12mg of Mg²⁺ ions. What is the volume of a 2N Na₂CO₃ solution required to soften 5000 L of pond water?

Milli- equivalents of Ca²⁺+ milli-equivalents of Mg²⁺
= milli-equivalents of ${\mathrm{Na}}_2{\mathrm{CO}}_3$:
$\frac{20}{40}+\frac{12}{24}=\mathrm{V}\times 2 \left(\because \mathrm{milli} \mathrm{equivalents}=\frac{\mathrm{Wt}.}{\mathrm{E}}\right)$
$\therefore$V = $\frac{1}{2}=0.5\text{ ml /lit}$

For 5000 L of pond water
$$\begin{gathered} \mathrm{V}=5000\times 0.5 =2500 \mathrm{ml} \\ =2.5 \mathrm{lit} \end{gathered}$$