Question

My lab is calling for me to calculate the hypochlorite molarity in bleach. The actual question is as follows:

Determine the average hypochlorite molarity in each bleach sample based on the molarity and volume of standardized thiosulfate solution to reach the end point, the overall reaction stoichiometry, and the volume of the sample tritrated.

I know the molarity and volume of the standardized thiosulfate solution (which is 0.157 M and 7.75 mL respectively) And I also know the volume of the sample titrated which is 14 mL) but I am having trouble determining the overall reaction stoichiometry and how that relates to the problem. I guess my question is how do I set up the equation to determine the hypochlorite molarity in bleach with the stoichiometry I am having trouble with and the known values.

Answer 1

we know that

moles = molarity x volume (L)

so

moles of S2032- = 0.157 x 7.75 x 10-3

so

moles of S2032- = 1.21675 x 10-3

now

the reactions are

1) NaOCl(aq) + HCl(aq) --> HOCl(aq) + NaCl(aq)

2)

HOCl (aq)+ HCl (aq)+ 3I (aq) ---> (I3)- (aq) +2Cl- (aq)+ H2O(l)

3)

[I3-][starch]+ 2S2O32- ---> 3I- + S4062- + starch

4)

the ultimate reaction is

H+ (aq) + HOCl(aq) + 2S2032- (aq) --> S4O62- (aq) + Cl- (aq) + H2O(aq)

we can see that

moles of HOCl = 0.5 x moles of S2032-

so

moles of HOCl = 0.5 x 1.21675 x 10-3

moles of HOCl = 6.08375 x 10-4

now

molarity = moles x 1000 / volume (ml)

so

molarity of HOCl = 6.08375 x 1000 / 14

molarity of HOCl = 0.043455

so

the molarity of hypochlorite is 0.043455 M