A 2.00 g sample of bleaching powder is shaken with water, and excess KI and acetic acid are added. The liberated iodine needs 42.2 mL of 0.250 M Na2S2O3 solution. Taking Cl2 + 2I- -> 2Cl- + I2 and I2 + 2S2O3^2- -> 2I- + S4O6^2-, and atomic mass of Cl = 35.5, calculate the available chlorine as per cent by mass of the sample (two decimals).

JEE Main levelNumerical answerDifficulty 2 of 3ph028

A 2.00 g sample of bleaching powder is shaken with water, and excess $KI$ and acetic acid are added. The liberated iodine needs 42.2 mL of 0.250 M $Na X_{2} S X_{2} O X_{3}$ solution. Taking $Cl X_{2} + 2 I X^{-} 2 Cl X^{-} + I X_{2}$ and $I X_{2} + 2 S X_{2} O X_{3} X^{2 -} 2 I X^{-} + S X_{4} O X_{6} X^{2 -}$ , and atomic mass of Cl = 35.5, calculate the available chlorine as per cent by mass of the sample (two decimals).

Numerical answer type, answer in %, tolerance 0.01.

Moles of thiosulfate = 0.250 x 0.0422 = 0.01055 mol. One mole of

Cl X_{2}

gives one mole of

I X_{2}

, which needs 2 mol thiosulfate, so mol

Cl X_{2}

= 0.005275. Mass of

Cl X_{2}

= 0.005275 x 71 = 0.3745 g, and % available chlorine = 0.3745/2.00 x 100 = 18.73.