5.00 mL of a commercial hypochlorite bleach is diluted to 250 mL. A 25.0 mL aliquot is added to excess acidified KI (OCl- + 2I- + 2H+ -> Cl- + I2 + H2O) and the iodine needs 21.5 mL of 0.0500 M Na2S2O3. Find the concentration of NaOCl (74.5 g/mol) in the original bleach in g/L (nearest integer).

JEE Advanced levelNumerical answerDifficulty 3 of 3eq163

5.00 mL of a commercial hypochlorite bleach is diluted to 250 mL. A 25.0 mL aliquot is added to excess acidified KI ( $OCl X^{-} + 2 I X^{-} + 2 H X^{+} Cl X^{-} + I X_{2} + H X_{2} O$ ) and the iodine needs 21.5 mL of 0.0500 M $Na X_{2} S X_{2} O X_{3}$ . Find the concentration of NaOCl (74.5 g/mol) in the original bleach in g/L (nearest integer).

Numerical answer type, answer in g/L, tolerance 0.6.

Thiosulfate = 0.0500

\times

21.5 = 1.075 mmol, so

I X_{2}

= 0.5375 mmol =

OCl X^{-}

in the aliquot. The 250 mL holds 5.375 mmol, all from 5.00 mL, so original concentration = 1.075 M. Mass concentration = 1.075

\times

74.5 = 80.09 g/L.