Question: 80g of h2 is reacted with 80g of o2?
Answer: The weight of unreacted H2=70 gm. The reaction is 2H2 + O2 gives 2H2O. That is one mole O2 requires two moles of hydrogen. And one mol H2 requirs half moles of O2. We have 80/2=40 mol H2 and 80/32=2.5 moles O2.