When looking at the passage and the 5 reactions given, it seems the ratio of moles of thiosulfate to oxygen would be 2:1, however that is not the case. And the explanation goes through a conversion.
I don't understand. Can you please explain?
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This question is probably easier to answer using the method you mention here -- looking at the 5 reactions they give you. The conversion they work with is tedious and long.
However, you have to be careful when you use these reactions. Reaction 2 is different from the rest -- because it has 2 Mn(OH)2 in it, and the rest only work with 1 Mn(OH)2. So, to make the reactions all equivalent, let's just multiply Reactions 1, 3, 4 and 5 by 2. This way, we can imagine that we start with 1 mol O2, and make 2 mol MnO(OH)2. Those 2 mol Mn(OH)2 make two mol Mn(SO4)2 (Reaction 3), which go on to make 2 mol I2 (Reaction 4), which finally are titrated with 4 mol S2O3(2-) (Reaction 5). Or, just jumping from Reaction 2 to Reaction 5, we have 1 mol O2 that we start with and, now that we've adjusted, we get out 4 mol thiosulfate. This gives us that 4:1 ratio they have as the answer.
This is similar to a mistake many people make in biology, actually -- in the area of cell respiration. Remember how 1 glucose makes 2 pyruvate? And then the reactions from then are are written with just 1 pyruvate? It's the same idea. We have to be consistent with the Reactions here.
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