Question

The amount of I3–(aq) in a solution can be determined by titration with a solution containing a known concentration of S2O32–(aq) (thiosulfate ion). The determination is based on the net ionic equation. Given that it requires 37.9 mL of 0.360 M Na2S2O3(aq) to titrate a 15.0-mL sample of I3–(aq), calculate the molarity of I3–(aq) in the solution.

Answer 1

The net ionic equation is---

2S₂O₃^2-(aq) + I₃^-(aq) S₄O₆^2-(aq) + 3I^-(aq)

37.9 mL of 0.360 M Na₂S₂O₃ = 0.360 M x 0.0371 L = 0.013644 mol S₂O₃^2-

It is evident from the above net ionic equation that 2 mol S2O3^2- reacts with 1 mol I₃^-
So, 0.013644 mol of S₂O₃^2- reacts with = 1/2 *0.013644 = 0.006822 mol I₃^-
Hence, 15.0 mL I₃^- containing 0.006822 mol will give a solution of 0.006822 mol /0.0150 L = 0.4548 M
Required molarity of I₃^- (aq) in the solution= 0.4548 M