So,
Given the reaction:
We want to find the theoretical yield of the reaction of aluminum oxide when 15.0 g of aluminum reacts with excess oxygen. To do this, we use the coefficients of the reaction and first, we pass all the amount in grams to moles. Then, we multiply by the molar mass in order to find the grams of Al2O3:
The theoretical yield, is 28.3g of Al2O3.