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Question:

Bleaching powder and bleach solution are produced on a large scale and is 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 Na2S2O3 was used to reach the end point. The molarity of the household bleach solution is:

0.24 M

0.024 M

0.48 M

0.96 M

Solution:

Ca(OCl)Cl + 2CH3COOH → Cl2 + (CH3COO)2Ca + H2O
Cl2 + 2KI → 2KCl + I2
I2 + 2Na2S2O3 → 2NaI + Na2S4O6
Meq. of I2 = Meq. of Cl2
∴ meq. of Na2S2O3 = 48 × 0.25
Millimoles of I2 reacted = 6
Millimoles of Cl2 reacted = 6
Millimoles of bleach = 6
25 × M = 6
M = 6/25 = 0.24 M