sblock Elements 2 Question 15
16. The reagent(s) used for softening the temporary hardness of water is(are)
(2010)
(a) $\mathrm{Ca}{3}\left(\mathrm{PO}{4}\right)_{2}$
(b) $\mathrm{Ca}(\mathrm{OH})_{2}$
(c) $\mathrm{Na}{2} \mathrm{CO}{3}$
(d) $\mathrm{NaOCl}$
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Answer:
Correct Answer: 16. (b)
Solution:
- $\mathrm{Ca}(\mathrm{OH}){2}+\mathrm{Ca}\left(\mathrm{HCO}{3}\right){2} \longrightarrow 2 \mathrm{CaCO}{3} \downarrow+\underset{\text { (Clark’s method) }}{2 \mathrm{H}_{2} \mathrm{O}}$
$$ \begin{aligned} \mathrm{NaOCl}+\mathrm{H}{2} & \longrightarrow \mathrm{NaOH}+\mathrm{HOCl} \ \mathrm{HO}^{-}+\mathrm{HCO}{3}^{-} & \longrightarrow \mathrm{CO}{3}^{2-}+\mathrm{H}{2} \mathrm{O} \ \mathrm{Ca}\left(\mathrm{HCO}{3}\right){2}+\mathrm{Na}{2} \mathrm{CO}{3} & \longrightarrow \mathrm{CaCO}{3} \downarrow+2 \mathrm{NaHCO}{3} \end{aligned} $$
