How many moles and grams of aluminum oxide are formed by the reaction of 6.75 g of aluminum with oxygen?

Let’s compose the equation for the reaction of interaction of aluminum with oxygen:
4Al + 3O2 = 2Al2O3;

Let’s find the amount of substance in aluminum:
6.75 / 27 = 0.25 mol;

There is 2 times less coefficient before aluminum oxide than before aluminum. This means that the amount of substance in aluminum oxide is 0.125 mol.
Let’s find the molecular weight of aluminum oxide:
27 * 2 + 16 * 3 = 102 g / mol;

Let’s find the mass of aluminum oxide:
102 * 0.125 = 12.75 grams;

Answer: the amount of substance in aluminum oxide is 0.125 mol; the mass of aluminum oxide is 12.75 grams.