[bayas]

on a test there was this problem:
2 SO2 + O2 <----> 2 SO3
there are 2 moles of SO2 and 1.5 moles of O2 before the reaction, after the reaction we have 1 mole of SO2, 1 mole of O2 and 1 mole of SO3, so like in this scheme:
2 SO2 + O2 <-----> 2 SO3
   2        1.5               0
   -1       -0.5              -1
   1          1                1

Now we where asked what the limiting reagent is, so I wrote up SO2, since O2 will be in excess if we use up all the moles of SO2. The teacher however doesn't agree with me and thinks that O2 is the limiting reagent because there are less moles (1,5) of O2 available. Is this true? And why is it true? And, if I'm right: how can I convince the teacher?

[Borek]

If your teacher thinks oxygen is limiting reagent looks like you have to fire your teacher and look for a new one

But what you have wrote doesn't make sense too - after the reaction there is no SO2 left (unless you are talking about some more complex equilibrium which I doubt). There are two moles of SO3 and half mole of oxygen left.

Try to explain that AFTER the reaction only oxygen is left (0.5 mole) from substrates, so if it is left it is obviously in excess and the SO2 is limiting.

[bayas]

Yes, we had the exercise as an exercise on chemical equilibria, here the volume was 5l and the K=5l/mol. We had to calculate the yield (for which we need the limiting reagent). thanks anyway and I'll try to convince her, but I don't think it'll work