sblock Elements 1 Question 13
9. The hardness of a water sample (in terms of equivalents of $\mathrm{CaCO}_{3}$ ) containing is
(Molar mass of $\mathrm{CaSO}_{4}=136 \mathrm{~g} \mathrm{~mol}^{-1}$ ) (2019 Main, 12 Jan I)
(a) $100 \mathrm{ppm}$
(b) $10 \mathrm{ppm}$
(c) $50 \mathrm{ppm}$
(d) $90 \mathrm{ppm}$
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Solution:
- Hardness of water sample can be calculated in terms of ppm concentration of $\mathrm{CaCO}_{3}$.
Given, molarity $=10^{-3} \mathrm{M}$
i.e. $1000 \mathrm{~mL}$ of solution contains $10^{-3}$ mole of $\mathrm{CaCO}_{3}$.
$\therefore$ Hardness of water $=$ ppm of $\mathrm{CaCO}_{3}$
$$ \begin{aligned} & =\frac{10^{-3} \times 1000}{1000} \times 10^{6} \ & =100 \mathrm{ppm} \end{aligned} $$
