A sample of a new cleaning product Joes Famous Bleach Cleane

A sample of a new cleaning product, \"Joe\'s Famous Bleach Cleaner,\" with a mass of 67.0 g , was diluted with an acetic acid solution containing excess I. A small amount of starch indicator solution was then added, turning the solution a deep bluish-purple. The solution was then titrated with 0.320 M sodium thiosulfate, Na2S2O3, containing the ion S2O32. A volume of 36.0 mL of sodium thiosulfate, the titrant, was needed to turn the solution colorless. What is the percentage composition by mass of NaClO in the bleach product? Express your answer to three significant figures and include the appropriate units.

Solution

The following sequence of reactions takes place during the whole process:

ClO- + I- + H2O ==> Cl- + I2 + 2OH-

I2 + I- ==> I3^-

I3^- + starch ==> I3^- - starch

The I3^- - starch is titrated with S2O3 2- until the blue color is absent.

I3^- - starch + 2S2O3 2- ==> 3I- + S4O6 2- + starch

On summation of all the reactions, the net reaction will be:

ClO- + 2S2O3 2- + H2O ==> Cl- + S4O6 2- + 2OH-

moles S2O3 2- = M S2O3 2- x L S2O3 2-

= (0.320)(0.0360)

= 0.01152 moles S2O3 2-

g NaClO = 0.01152 moles S2O3 2- x (1 mole ClO- / 2 moles S2O3 2-) x (1 mole NaClO / 1 mole ClO-) x (74.5 g NaClO / 1 mole NaClO) = 0.429 g NaClO

%NaClO = (g NaClO / g bleach) x 100

= (0.429 / 67.0) x 100

= 0.640 % NaClO