Question

To find the concentration of bleach, you will need to back-calculate from the amounts of other reagents used.

If 9.8ml of 0.36M sodium thiosulfate were used in the titration, calculate

A. Moles of KI3 generated

B. Moles of KI

C. Moles HOCl

D. Moles of NaOCl

Mass percent is defined as: mass % = mass of NaOCl/Mass of bleach solution x 100%

If you know the mass % of bleach is 4.5%, then what mass of active ingredient do you expect from 20ml of bleach solution?

Cl2 + 2 NaOH --> <-- NaCl + H2O + NaOCl

HOCl + 3KI + HCl --> 2KCl + H2O + KI3

KI3 + 2Na2S2O3*5H2O -----------> Na2S4O6 + KI + 2NaI + 5H2O

6.25 percent sodium hypochlorite

.05 percent sodium hydroxide

Answer 1

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First you have to calculate the number of mol of thiosulfate used in the titration. This number of moles can be related to the amount of KI₃ through the titration reaction:

n(Na₂S₂O₃) = 9.8 mL x 0.36 mmol/mL = 3.528 mmol of Na₂S₂O₃

Now, through the titration equation we see that 1 mol of KI₃ requires 2 moles of Na₂S₂O₃ to react. This means that the numbers of moles of KI₃ present are half the number of moles of Na₂S₂O₃.

N(KI₃) = 3.528 mmol / 2 = 1.764 mmol

To calculate the number of moles of KI we have to use the equation:

HOCl + 3KI + HCl --> 2KCl + H₂O + KI₃

This equation relates KI₃ with the KI necessary to form the potassium triiodide. We can see that there are needed three moles of KI to form one mol of potassium triiodide. So the number of moles of KI is:

N(KI) = 1.764 mmol x 3 = 5.292 mmol of KI

On the other hand and using the same equation we can see that each mol of potassium triiodide formed comes from one mol of HOCl. So the number of moles of HOCl is exactly the same as the number of moles of potassium triiodide. Also the number of moles of NaOCl should be the same since for each mol of NaOCL a mol of HOCl is produced upon acidification.

Mas of bleach or active ingredient: bleach is NaOCl so you have to determine the amount of NaOCl in 20 mL of bleach solution. Assuming a density of 1 g/mL because these are aqueous solutions we apply the equation they gave you clearing for the mass of NaOCl:

Mass percent is defined as: mass % = mass of NaOCl/Mass of bleach solution x 100%

Mass of NaOCl = mass% x Mass of bleach solution /100% = 4.5% x 20 g / 100% = 0.9 g of NaOCl

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