TEST OF PRACTICAL KNOWLEDGE QUESTION All your burette readings (initials and final) as well as the size of your pipette must be recorded but no account of e...

TEST OF PRACTICAL KNOWLEDGE QUESTION

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 dm\(^3\) of Na\(_2\)S\(_2\)O\(_3\). B was obtained by dissolving 9.0 g of an impure sample of I\(_2\) in aqueous Kl and the solution made up to 1 dm\(^3\).

(a) Put A into the burette and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) 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 I\(_2\) + 2S\(_2\)O\(_3\) \(\to\) 2I\(^-\) + S\(_4\)O\(_6^{2-}\).

(b) From your results and the information provided, calculate the:

(i) concentration of A in mol dm\(^{-3}\) 

(ii) concentration of I\(_2\) in B in mol dm\(^{-3}\);

(iii) percentage by mass of I\(_2\) 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.

Answer Details

Titration Experiment with Sodium Thiosulfate and Iodine

Experiment:

You are titrating a solution of sodium thiosulfate (Na2S2O3, solution A) against a solution containing iodine (I2, solution B) obtained from an impure sample.

Requirements:

• Record initial and final burette readings for titrating solution A against B (20.0 cm3 or 25.0 cm3 portions).
• Use starch solution as an indicator (not added at the beginning).
• Repeat the titration for concordant titers (consistent volumes used).

(a) Average Volume of Solution A:

1. From your recorded burette readings, calculate the volume of solution A used in each titration.
2. Perform multiple titrations and discard any outliers.
3. Calculate the average volume of solution A used for all concordant titrations.

(b) Concentration Calculations:

i) Concentration of Solution A (Na2S2O3):

1. Use the provided concentration of Na2S2O3 in solution A (15.8 g dm-3).
2. Convert the concentration from g dm-3 to mol dm-3 using the molar mass of Na2S2O3 (248.2 g mol-1).

ii) Concentration of Iodine (I2) in Solution B:

1. Use the balanced equation for the reaction: I2 + 2S2O3- → 2I- + S4O6-2
2. The moles of Na2S2O3 used are equal to the moles of I2 reacted (from stoichiometry).
3. Calculate the moles of Na2S2O3 used from the concentration of solution A and the average volume used.
4. Divide the moles of Na2S2O3 by the volume of solution B used (0.0200 dm3 or 0.0250 dm3 depending on your chosen volume) to get the concentration of I2 in mol dm-3.

iii) Percentage by Mass of I2 in the Sample:

1. Calculate the total moles of I2 present in 1 dm3 of solution B using the concentration obtained in (b)(ii).
2. Multiply the moles of I2 by the molar mass of I2 (254.0 g mol-1) to get the total mass of I2 in 1 dm3 of solution B.
3. Divide the mass of I2 by the mass of the impure sample used (9.0 g) and multiply by 100% to get the percentage by mass of I2 in the sample.

(c) Why Not Add Starch Indicator at the Beginning?

Starch forms a blue-black complex with free iodine molecules. If added at the beginning, the solution would appear blue even before the endpoint. By adding starch near the endpoint (when free I2 concentration is low), a faint blue color appears, indicating the complete reaction of I2 with Na2S2O3. This allows for a more accurate determination of the titration endpoint.