Sodium thiosulfate, Na2S2O3, is used as a fixer in photographic film developing. The amount of Na2S2O3 in a solution can be determined by a titration with iodine, I2, according to the equation: 2Na2S2O3(aq) + I2(aq) --> Na2S4O6 +2NaI(aq).

Question

Calculate the concentration of the Na2S2O3 solution if 21.40 mL of a 0.1350 M I2 solution react exactly with a 100.0 mL sample of the Na2S2O3 solution. Use 4 sig. fig.

Aside, the end of the titration is determined by the color. NaI is a pale yellow and I2 is a deep purple. Just when the purple color persists, all the iodine that can react has reacted.

Answer 1

2S2O3^2- + I2 ==> 2I^- + S4O6^2-
1. mols I2 = M x L = ?
2. Convert mols I2 to mols S2O3^2-. That is ?mols I2 x (2 mol S2O3^2-/1 mol I2) = ?
3. Then M S2O3^2- = mols S2O3^2-/L S2O3^2-