Question

Given the balanced equation representing a reaction: 4Al(s) + 3O2(g) → 2Al2O3(s) How many moles of Al(s) react completely with 4.50 moles of O2(g) to produce 3.00 moles of Al2O3(s)?

Answer 1

We must consider the relation between Al and $O_2$:

$\frac{3\ mol\ O_2}{4\ mol\ Al} = \frac{4,5\ mol\ O_2}{x}\ \to\ x = \frac{4,5\ mol\ O_2\cdot 4\ mol\ Al}{3\ mol\ O_2} = \bf 6\ mol\ Al$

As you can see, the number of moles of Al is double to the number of moles of $Al_2O_3$, keeping constant the proportion of both substances.