Chemistry Winkler Titration help?

Question

Given these equations in order:
4MnSO4 + 8KOH --> 4Mn(OH)2
4Mn(OH)2 + O2 + H2O --> 4Mn(OH)3
4Mn(OH)3 + 6H2SO4 --> 2Mn2(SO4)3 + 12H2O
2 Mn2(SO4)3 + 4 KI --> 4MnSO4 + 2K2SO4 + 2I2
2I2 + 2I- --> 2I3-
3Na2S2O3 + 2I3- --> Na2S4O6 + 4 Na+ + 6 I-

and given Molarity of Na2S2O3 is .0000855 mol/L, how many moles of O2 are present?

Tell me what stoichiometry steps to work with this please!

Actually .0000855 is the number of moles, NOT the molarity

Answer 1

Work backwards.

From moles of Na2S2O3 you can find moles of I3-, which is equal to moles of I2, which gives you moles of Mn3+, and so on.

The stoichiometry that you need is simply the stoichiometry of the reaction that you have written. For example,

4 moles Mn(OH)3 correspond to 2 moles O2

You can do this step-by-step, working out the concentrations of all the intermediates, or you can do it in a single step by writing out all the conversion factors.

You obviously meant to assume that you use excess Mn2+, OH-, and H+. OH- and H+ do not directly enter into the redox reactions, and you use excess Mn2+ so that O2 is the limiting reagent.