Sodium thiosulfate, Na2S2O3, is used as a fixer in photographic film developing.

Question

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).

Calculate the concentration of the Na2S2O3 solution if 47.60 mL of a 0.2910 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.

Explanation / Answer

2Na2S2O3(aq) + I2(aq) --> Na2S4O6 +2NaI(aq)

concentration of the Na2S2O3 solution =?

Volume of I2 solution = 47.60 mL

Concentration of I2 solution = 0.2910 M

volume of Na2S2O3 solution = 100 mL = 0.100 L

Moles of I2 = concentration x volume

= 0.2910 mol/L x 47.60 mL x 1L/1000 mL

= 0.0138516 mol

From the stoichiometry of the reaction

Moles of Na2S2O3 required

= 2 mol Na2S2O3 x 0.0138516 mol I2 / 1 mol I2

= 0.0277032 mol Na2S2O3

concentration of the Na2S2O3 solution

= 0.0277032 mol / 0.100 L

= 0.27703 M

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