IIT-JEE 2012 · Paper 2 · Chemistry Q30

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Paragraph for Questions 29 and 30

Bleaching powder and bleach solution are produced on a large scale and used in several household products. The effectiveness of bleach solution is often measured by iodometry.

25 mL of household bleach solution was mixed with 30 mL of 0.50 M KI and 10 mL of 4 N acetic acid. In the titration of the liberated iodine, 48 mL of 0.25 N $\ce{Na2S2O3}$ was used to reach the end point. The molarity of the household bleach solution is

0.48 M

0.96 M

0.24 M

0.024 M

Answer and worked solution

Official answer: C

Reactions.
$\ce{OCl- + 2I- + 2H+ -> Cl- + I2 + H2O},\qquad\ce{I2 + 2S2O3^{2-} -> 2I- + S4O6^{2-}}.$
So 1 mol of hypochlorite ≡ 1 mol $\ce{I2}$ ≡ 2 mol thiosulphate.

Amounts. Thiosulphate: $0.048\times0.25=0.012$ mol. Hypochlorite: $0.006$ mol in 25 mL.

Molarity. $\dfrac{0.006}{0.025}=0.24$ M. (KI is in excess: 0.015 mol.)

(C)

Topics:Redox titrations Stoichiometry

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IIT-JEE 2012 · Paper 2, Chemistry question 30