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,c,d)
Solution:
- $\mathrm{Ca}(\mathrm{OH}) _{2}+\mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2} \longrightarrow 2 \mathrm{CaCO} _{3} \downarrow+\underset{\text { (Clark’s method) }}{2 \mathrm{H} _{2} \mathrm{O}}$
$$ \begin{aligned} \mathrm{NaOCl}+\mathrm{H} _{2} & \longrightarrow \mathrm{NaOH}+\mathrm{HOCl} \\ \mathrm{HO}^{-}+\mathrm{HCO} _{3}^{-} & \longrightarrow \mathrm{CO} _{3}^{2-}+\mathrm{H} _{2} \mathrm{O} \\ \mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2}+\mathrm{Na} _{2} \mathrm{CO} _{3} & \longrightarrow \mathrm{CaCO} _{3} \downarrow+2 \mathrm{NaHCO} _{3} \end{aligned} $$
