Harris Chapters 10, 11 and 15

1. What is a Gran plot used for? Your answer should include a description of why it is used, what data is collected, and a drawing of what is plotted (including the determination of Ve).

2. A dibasic, weak base (B), with pKb1 = 4.00 and pKb2 = 8.00, was titrated with 1.00 M HCl. The initial solution of B was 0.100 M and had a volume of 100.0 mL. Find the pH at the following volumes of acid added and make a graph of the pH versus Va: Va = 0, 5, 10, 15, 20, and 22 mL.

3. Methods involving Iodine

(a) Potassium iodate solution was prepared by dissolving 1.052 g of KIO3 (FM 214.00) in a 500 mL volumetric flask. Then 50.00 mL of the solution was pipetted into a flask and treated with excess KI (2 g) and acid (10 mL of 0.5 M H2SO4). How many moles of I3- are created by the reaction?

(b) The triiodide from part (a) reacted with 39.25 mL of Na2S2O3 solution. What is the concentration of the Na2S2O3 solution?

(c) A 1.223 g sample of solid containing ascorbic acid and inert ingredients was dissolved in dilute H2SO4 and treated with 2 g of KI and 50.00 mL of KIO3 solution from part (a). Excess triiodide required 14.58 mL of Na2S2O3 solution from part (b). Find the weight percent of ascorbic acid (FM 176.13) in the unknown.

(d) Does it matter whether starch indicator is added at the beginning or near the end point in the titration in part (c)?Explain.

Starch solution should not be added until just before the end point because I3- is present throughout

the titration and will irreversibly bind to starch if starch is added too early.

4. Ascorbic acid (0.0100 M) was added to 10.0 mL of 0.0200 M Fe3+ in a solution buffered to pH 0.30, and the potential was monitored with Pt and saturated Ag | AgCl electrodes. The half reaction for reduction of ascorbic acid is:

dehydoascorbic acid + 2 H+ + 2 e- ascorbic acid + H2O E° = 0.390 V

(a) Write a balanced equation for the titration reaction. Use the lowest possible coefficients.

(b) Using E° = .767 V for the Fe3+ | Fe2+ couple, calculate the cell voltage when 4.9, 10.0, and 15.4 mL of ascorbic acid have been added.

(c) At what potential is the equivalence point?