A lump of impure iodine, I2, of mass 0.4000 g was ground to powder and dissolved in potassium iodide, KI, solution. The solution was made up to 250.00 cm^3. 25.00 cm^3 of this solution was then titrated with 0.01 mol dm^-3 sodium thiosulfate (Na2S2O3) solution which converted to S4O6^2-. The mean titre was 26.00 cm^3.
(a) Give the oxidation states of sulfur in S2O3^2- and in S4O6^2-.
(b) Write balanced half-equations for this reaction.
(c) Write the overall redox equation for this reaction.
(d) Name the oxidizing and reducing agent of this reaction.
(e) How many moles of sodium thiosulfate were used in the titration?
(f) Calculate the number of moles of iodine, I2, in 25.00 cm^3 of solution.
(g) Calculate the number of moles of iodine in 250.00 cm^3 of solution.
(h) Calculate the mass of iodine present in part (g).
(i) Calculate the percentage purity of iodine.
(j) How would you expect the presence of an air bubble in your burette to impact your percentage purity calculation in part (i)? Would it lead to an overestimation or an underestimation of your calculated value? Explain your answer.
