Chemistry Question

Question

The amount of I−3(aq)I3−(aq) in a solution can be determined by titration with a solution containing a known concentration of S2O2−3(aq)S2O32−(aq) (thiosulfate ion). The determination is based on the net ionic equation

2S2O2−3(aq)+I−3(aq)⟶S4O2−6(aq)+3I−(aq)2S2O32−(aq)+I3−(aq)⟶S4O62−(aq)+3I−(aq)

Given that it requires 42.2 mL42.2 mL of 0.310 M Na2S2O3(aq)0.310 M Na2S2O3(aq) to titrate a 30.0 mL30.0 mL sample of I−3(aq),I3−(aq), calculate the molarity of I−3(aq)I3−(aq) in the solution.

Answer 1

2S₂O₃^2- + I₃^- ==> S₄O₆^2- + 3I^-

moles S₂O₃^2- = 42.2 ml x 1 L / 1000 ml x 0.310 mol / L = 0.01308 mols

moles I₃^- = 0.01308 mols S₂O₃^2- x 1 mol I3^- / 2 mol S₂O₃^2- = 0.006541 mols

Molarity of I₃^- solution = 0.006541 mols / 30.0 ml x 1000 ml / L = 0.218 moles / L = 0.218 M