Question

what is the mass of H2O produced when 10.0 grams of H2 reacts completely with 80.0 grams of O2?

Answer 1

molar mass of O2= 16.0 g/mol
molar mass of H2O= 18.0 g/mol

10.0 g of O2= 5 mol
80.0 g of H20= 5 mol

you need twice as the moles of H2 as O2 so there is insufficient H2 and it is the limiting reactant and all of it will react therefore 5.00 moles of H2 with 2.5 mole of O2 to produce 5.00 moles H2O

5.00 moles of H2O is 5.00x18.00=90.00 g

so the answer is 90.00