In the reaction Si + O2 -> SiO2 (∆H = -218 kcal/mol at 25˚C)...

In the reaction Si + O2 -> SiO2 (∆H = -218 kcal/mol at 25˚C), what is ∆H in kcal when 80 g of 02 reacts with 70g of Si?

80g O2 / 32g/mol O2 = 2.5 moles O2
70g Si / 28.09g/mol Si = 2.49199 moles Si

Because O2 and Si react at a 1:1 ratio, Si is the limiting reactant so 2.49199 moles of SiO2 are formed.

2.49199 x -218 = -543.25382 Kcal = DeltaH

70gr of Si
using this formula
[Mass of Si/Molar Mass of Si] = number of moles of Si
so 70/28 = 2.5 moles of Si

80g of Oxygen
using the same formula above to find the number of moles of oxygen
80/32 =2.5 moles of Si

Here there are no limiting reagents.
for every 1 mole of Si and O2 produces one mole of SiO2
you know that you have 2.5 moles of Si and O2 so you are going to get 2.5 moles of SiO2
and each mole yields -218kcal/mol
so 2.5 moles x -218kcal/mol = -545kcal