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All your burettereadings(initials and final) as well as the size of your pipette must ...

All your burette readings (initials and final) as well as the size of your pipette must be recorded but no account of experimental procedure is required. All calculations must be done in your answer booklet.
A is a solution containing 15.8 g dm3 of Na2S2O3. B was obtained by dissolving 9.0 g of an impure sample of I2 in aqueous Kl and the solution made up to 1 dm3.
(a) Put A into the burette and titrate it against 20.0 cm3 or 25.0 cm3 portions of B. Use starch solution as indicator. Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of A used. The equation for the reaction involved in the titration is I2 + 2S2O3 2I + S4O62.
(b) From your results and the information provided, calculate the:
(i) concentration of A in mol dm3
(ii) concentration of I2 in B in mol dm3;
(iii) percentage by mass of I2 in the sample
(c) Give reasons why the starch indicator was not added to the titration mixture at the beginning of the titration. [O = 16.0, Na = 23.0, S = 32.0, 1 = 127.0] Credit will be given for strict adherence to the instructions for observations precisely recorded and for accurate inferences. AIl tests, observations and inferences must be clearly entered in the booklet in ink at the time they are made.





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