IIT-JEE 2010 · Paper 1 · Chemistry Q9

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ChemistryMultiple correct+3Question 9 of 84

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

\ce{Ca3(PO4)2}

\ce{Ca(OH)2}

\ce{Na2CO3}

\ce{NaOCl}

Answer and worked solution

Official answer: B

Temporary hardness is due to calcium and magnesium bicarbonates. Clark's method adds slaked lime, which converts bicarbonate to insoluble carbonate:
$\ce{Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 v + 2H2O}.$

$\ce{Ca3(PO4)2}$ is insoluble and does nothing.
$\ce{Na2CO3}$ is the reagent for permanent hardness (chlorides and sulphates).
$\ce{NaOCl}$ is a disinfectant.

(B)

Topics:Water and hydrogen peroxide

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IIT-JEE 2010 · Paper 1, Chemistry question 9