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JEE Advanced · Chemistry · 10. Hydrogen

The reagent(s) used for softening the temporary hardness of water is(are)

  1. A \(\mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2\)
  2. B \(\mathrm{Ca}(\mathrm{OH})_2\)
  3. C \(\mathrm{Na}_2 \mathrm{CO}_3\)
  4. D \(\mathrm{NaOCl}\)
Verified Solution

Answer & Solution

Correct Answer

(B) \(\mathrm{Ca}(\mathrm{OH})_2\)

Step-by-step Solution

Detailed explanation

Softening the temporary hardness of water means conversion of bicarbonate into calcium carbonate (precipitation)
\(
\begin{gathered}
\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2+\mathrm{Ca}(\mathrm{OH})_2 \longrightarrow \\
2 \mathrm{CaCO}_3 \downarrow+2 \mathrm{H}_2 \mathrm{O} \\
\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2+\mathrm{Na}_2 \mathrm{CO}_3 \longrightarrow \\
\mathrm{CaCO}_3 \downarrow+2 \mathrm{NaHCO}_3
\end{gathered}
\)
Even \(\mathrm{NaOCl}\) can be used, as in aqueous medium \(\overline{\mathrm{O}} \mathrm{Cl}\) gets hydrolysed and makes solution alkaline enough to cause precipitation of \(\mathrm{CaCO}_3\).
\(
\begin{aligned}
\mathrm{NaOCl}+\mathrm{H}_2 \mathrm{O} & \rightleftharpoons \mathrm{HOCl}+\mathrm{NaOH} \\
\mathrm{OH}^{-}+\mathrm{HCO}_3^{-} & \longrightarrow \mathrm{CO}_3^{2-}+\mathrm{H}_2 \mathrm{O}
\end{aligned}
\)
Representative elements (Hydrogen and its compounds)
Conceptual
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