A square photographic film 2.00 cm on an edge was suspended in a 5% solution of Na2S2O3 to form a Ag(S2O3)2^3- complex and release the halide. After removal and washing of the film, the solution was treated with an excess of Br2 in basic solution to oxidize the I^- to IO3^- (and also to destroy the excess S2O3^2-). The solution was boiled to remove the Br2, and an excess of KI was added. The liberated I2 was titrated with 13.7 mL of 0.0352 M Na2S2O3 solution (the products are S4O6^2- and I^-).

Question

A square photographic film 2.00 cm on an edge was suspended in a 5% solution of Na2S2O3 to form a Ag(S2O3)2^3- complex and release the halide. After removal and washing of the film, the solution was treated with an excess of Br2 in basic solution to oxidize the I^- to IO3^- (and also to destroy the excess S2O3^2-). The solution was boiled to remove the Br2, and an excess of KI was added. The liberated I2 was titrated with 13.7 mL of 0.0352 M Na2S2O3 solution (the products are S4O6^2- and I^-).

a) Write balanced equations for the reactions of interest.
b) Calculate the mg of AgI per cm^2 of film.

--> I have no clue where my professor came up with the equations for part A, nor how he calculated the mg/cm^2.

Answer 1

To solve this problem, we need to go step by step.

a) First, let's write balanced equations for the reactions involved:

1) Formation of Ag(S2O3)2^3- complex:
AgI (s) + 2Na2S2O3 (aq) → Ag(S2O3)2^3- (aq) + 2NaI (aq)

2) Oxidation of I^- to IO3^-:
5I^- (aq) + 6Br2 (aq) + 6OH^- (aq) → 3IO3^- (aq) + 5Br^- (aq) + 3H2O (l)

3) Titration of liberated I2 with Na2S2O3:
I2 (aq) + 2S2O3^2- (aq) → 2I^- (aq) + S4O6^2- (aq)

It is important to know that the balanced equations are written based on the stoichiometry and the rules of conservation of mass. If you have any specific questions about how these equations were derived, feel free to ask.

b) To calculate the mg of AgI per cm2 of film, we need to use the given information and the equations. Here are the steps:

1) Calculate the number of moles of Na2S2O3 used in the titration:
moles of Na2S2O3 = volume of Na2S2O3 solution (L) × molarity of Na2S2O3

2) From the balanced equation 3, we know that 1 mole of Na2S2O3 reacts with 1 mole of I2. So, the moles of I2 liberated during the titration will be the same as the moles of Na2S2O3 used.

3) Calculate the mass of AgI using the stoichiometry from equation 1:
mass of AgI = moles of Na2S2O3 × molar mass of AgI

4) Finally, calculate the mass of AgI per cm2 of film:
mass of AgI per cm2 = mass of AgI / surface area of the film

Please provide the values of the volume of Na2S2O3 solution and the molarity of Na2S2O3, and I can help you calculate the final answer.