How much heat will be transferred?

Question

How much heat will be transferred when 0.54 mol sulfur reacts with 0.75 mol oxygen according to the following equation?

2S(s) + 3O2(g) -->2SO3(g)
enthalpy change=-790kJ

How do you calculate when there are no grams given to you? Do you use sulfur or oxygen?

Answer 1

You use the moles that are given, and you use the balanced equation to find the limiting reagent. First off:

0.54molS x 3molO2/2molS = (0.54)(3)/(2) = 0.81 molO2 will be needed to react with 0.54molS

But there is only 0.75molO2 available Therefore, O2 is the limiting reagent. Start there:

0.75molO2 x

But 790kJ is the enthalpy change per what?