In the reaction 2A+B2 to 2AB if 0.5 moles of A and an excess of B are combined and the reaction goes to %100 completion, then how many moles of C are formed?
There is no C in the original equation, so I'll assume that you mean C is AB.
Then there would be 0.5 moles, since A is the limiting reagent, and the mole ratio of A to AB is 1:1 (or 2:2 for the sake of understanding it).
Hope this helps. Like I said, its a bit hard to know, if there is no explanation of what C is.