Question 1
A is a solution containing 15.8 g dm-3 of Na2S2O3.
B was obtained by dissolving 9.0 g of an impure sample of I2 in aqueous KI and the solution made up to 1 dm3.
- Put A into the burette. Pipette 20.0 cm3 or 25.0 cm3 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 + 2S2O32- → 2I- + S4O62- -
From your results and information provided, calculate the:
- concentration of A in moldm-3;
- concentration of I2 in B in mol dm-3;
- percentage by mass of I2 in the sample.
-
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, I = 127.0 ]
Observation
This question was attempted by all the candidates, and the performance was good.
In part (a), candidates carried out the titration with correct averaging. However, some lost marks due to alteration, inconsistency in decimal places of burette readings and wrong averaging.
In part (b), majority of the candidates were able to calculate the concentrations of Na2S2O3 and I2 in moldm-3.
In part (c), majority of the candidates could not give the reason why the starch indicator was not added to the titration mixture at the beginning of the titration.
The expected answers include:
