In the reaction 2H2S + 3O2 -> 2SO2 +2H2O how many moles of O2 are necessary to react with 0.75 moles of H2S?

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For these kinds of questions, make sure that the chemical equation is balanced. In this case, it already is balanced. 

What follows, then, would simply be stoichiometry.

We are given the number of moles of `H_2S` and we would want to know the amount of `O_2` needed to react with the given. 

Using the chemical equation, notice that 2 moles of `H_2S`  reacts with 3 moles of `O_2`  to produce the products. Hence, the molar ratio of `H_2S` to `O_2`  is 2:3 -- in other words, 2 moles of hydrogen sulfide is ''equivalent'' to 3 moles of oxygen gas. Hence:

`0.75 mol H_2S times (3 mol O_2)/(2 mol H_2S) = 1.125 mol O_2`

We need 1.125 moles of oxygen gas to react with the given amount of hydrogen sulfide.

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