A student weighs 1.085 g of bleach into a flask to which 2.000 g of KI and 10 mL

Question

A student weighs 1.085 g of bleach into a flask to which 2.000 g of KI and 10 mL of 1.0 M H2SO4 are also added. The solution is mixed and titrated with a 0.1221 M thiosulfate solution. The initial burette reading is 5.00 mL and the final burette reading is 26.65 mL. Use this information to answer all prelab questions.

1.What net volume of thiosulfate is used in the titration of the bleach sample?

2.How many moles of thiosulfate were used to titrate the bleach sample?

3.What is the mole ratio of thiosulfate to hypochlorite in this experiment?

4.How many moles of hypochlorite were titrated by the thiosulfate?

5.What is the mass of sodium hypochlorite in the sample of bleach?

6.What is the weight percent (%) of sodium hypochlorite in the bleach sample?

Explanation / Answer

OCl- + 2 I- + 2 H+ => I2 + Cl- + H2O

I2 + 2 S2O3- => S4O62- + 2 I-


(1) Net volume of S2O32- = final - initial burette reading

= 26.65 - 5.00 = 21.65 mL


(2) Moles of S2O32- = volume x concentration of S2O32-

= 21.65/1000 x 0.1221 = 0.002643 mol


(3) Moles of S2O32- : I2 = 2 : 1

Moles of I2 : OCl = 1 : 1

=> Moles of S2O32- : moles of OCl- = 2 : 1

i.e. mole ratio is 2


(4) Moles of OCl- = 1/2 x moles of S2O32-

= 1/2 x 0.002643 = 0.001322 mol


(5) Mass of NaOCl = moles of OCl- x molar mass of NaOCl

= 0.001322 x 74.44 = 0.09839 g


(6) Weight% of NaOCl = mass of NaOCl/mass of sample x 100%

= 0.09839/1.085 x 100% = 9.068%

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