Assume that a 2.130g sample of bleach was weighed out as outlined in the procedu

Question

Assume that a 2.130g sample of bleach was weighed out as outlined in the procedure. When a sodium thiosulfate solution containing 6.829g of Na2S2O35H2O in 250.0mL was used to titrate the above bleach sample, it required 11.21mL to reach the starch endpoint.

            a) What is the molarity of the sodium thiosulfate pentahydrate solution?

            b) What is the number of moles of Na2S2O3 in the 11.21mL?

            c) What is the number of moles of I2 that react with the Na2S2O3?

            d) How many moles of NaOCl are in the bleach sample? How many grams?

            e) What is the percent by mass of NaOCl in the bleach?

Explanation / Answer

molar mass of Na2S2O35H2O = 248.2

a) the molarity of the sodium thiosulfate pentahydrate solution = 6.829g/248.2*0.25L = 0.11M

b) the number of moles of Na2S2O3 in the 11.21mL = 11.2*6.829/248.2*250 = 0.00123 moles

c) 1 mole of I2 + 2 moles of Na2S2O3 ==> 2 NaI + NaS4O6

the number of moles of I2 that react with the Na2S2O3 = 0.00123/2 = 0.000615 moles of I2

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