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 \(15.8\ \text{g dm}^3\) of \(\mathrm{Na_2S_2O_3}\). B was obtained by dissolving 9.0 g of an impure sample of \(\mathrm{I_2}\) in aqueous Kl and the solution made up to \(1\ \text{dm}^3\).
(a) Put A into the burette and titrate it against \(20.0\ \text{cm}^3\) or \(25.0\ \text{cm}^3\) 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 \(\mathrm{I_2 + 2S_2O_3 \to 2I^- + S_4O_6^{2-}}\).
(b) From your results and the information provided, calculate the:
(i) concentration of A in \(\text{mol dm}^{-3}\)
(ii) concentration of \(\mathrm{I_2}\) in B in \(\text{mol dm}^{-3}\);
(iii) percentage by mass of \(\mathrm{I_2}\) 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.
(a) Burette readings and average titre
| Burette reading (cm3) | Rough | 1st titration | 2nd titration | 3rd titration |
|---|
| Final reading | 12.60 | 25.10 | 12.40 | 25.00 |
| Initial reading | 0.00 | 12.60 | 0.00 | 12.40 |
| Volume of A used | 12.60 | 12.50 | 12.40 | 12.60 |
Average titre \(= \dfrac{12.50 + 12.40 + 12.60}{3} = 12.50\ \text{cm}^3\) of A, for a 25.0 cm3 portion of B.
Equation: \(I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}\).
(b)(i) Concentration of A in mol dm-3
Molar mass of Na2S2O3 \(= (2\times23) + (2\times32) + (3\times16) = 46 + 64 + 48 = 158\ \text{g mol}^{-1}\).
Concentration of A \(= \dfrac{15.8}{158} = 0.100\ \text{mol dm}^{-3}\).
(ii) Concentration of I2 in B in mol dm-3
Using \(\dfrac{C_A V_A}{C_B V_B} = \dfrac{n_A}{n_B} = \dfrac{2}{1}\), with \(C_A = 0.100\), \(V_A = 12.50\ \text{cm}^3\), \(V_B = 25.00\ \text{cm}^3\):
\(C_B = \dfrac{C_A V_A}{2 V_B} = \dfrac{0.100 \times 12.50}{2 \times 25.00} = \dfrac{1.25}{50} = 0.0250\ \text{mol dm}^{-3}\).
(iii) Percentage by mass of I2 in the sample
Molar mass of I2 \(= 2 \times 127 = 254\ \text{g mol}^{-1}\).
Concentration of I2 in g dm-3 \(= 0.0250 \times 254 = 6.35\ \text{g dm}^{-3}\).
The impure sample provides 9.0 g dm-3, so
\(\% I_2 = \dfrac{6.35}{9.0} \times 100 = 70.56\%\).
(c) Starch is not added at the beginning because, while the iodine concentration is still high, it forms a very stable dark blue-black iodine-starch complex that is only slowly decolorized. This would trap iodine and give a late, inaccurate end point. Starch is therefore added only near the end point, when the solution has faded to a pale straw-yellow colour, so that the end point is sharp and the titre value accurate.
(a) Burette readings and average titre
| Burette reading (cm3) | Rough | 1st titration | 2nd titration | 3rd titration |
|---|
| Final reading | 12.60 | 25.10 | 12.40 | 25.00 |
| Initial reading | 0.00 | 12.60 | 0.00 | 12.40 |
| Volume of A used | 12.60 | 12.50 | 12.40 | 12.60 |
Average titre \(= \dfrac{12.50 + 12.40 + 12.60}{3} = 12.50\ \text{cm}^3\) of A, for a 25.0 cm3 portion of B.
Equation: \(I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}\).
(b)(i) Concentration of A in mol dm-3
Molar mass of Na2S2O3 \(= (2\times23) + (2\times32) + (3\times16) = 46 + 64 + 48 = 158\ \text{g mol}^{-1}\).
Concentration of A \(= \dfrac{15.8}{158} = 0.100\ \text{mol dm}^{-3}\).
(ii) Concentration of I2 in B in mol dm-3
Using \(\dfrac{C_A V_A}{C_B V_B} = \dfrac{n_A}{n_B} = \dfrac{2}{1}\), with \(C_A = 0.100\), \(V_A = 12.50\ \text{cm}^3\), \(V_B = 25.00\ \text{cm}^3\):
\(C_B = \dfrac{C_A V_A}{2 V_B} = \dfrac{0.100 \times 12.50}{2 \times 25.00} = \dfrac{1.25}{50} = 0.0250\ \text{mol dm}^{-3}\).
(iii) Percentage by mass of I2 in the sample
Molar mass of I2 \(= 2 \times 127 = 254\ \text{g mol}^{-1}\).
Concentration of I2 in g dm-3 \(= 0.0250 \times 254 = 6.35\ \text{g dm}^{-3}\).
The impure sample provides 9.0 g dm-3, so
\(\% I_2 = \dfrac{6.35}{9.0} \times 100 = 70.56\%\).
(c) Starch is not added at the beginning because, while the iodine concentration is still high, it forms a very stable dark blue-black iodine-starch complex that is only slowly decolorized. This would trap iodine and give a late, inaccurate end point. Starch is therefore added only near the end point, when the solution has faded to a pale straw-yellow colour, so that the end point is sharp and the titre value accurate.
