Consider the reaction: 4 Al + 3O2 --> 2 Al2O3 How many moles of aluminum react with 2.1 moles of oxygen?

Answer 1

A balanced chemical reaction is important for a couple reasons:

1. It obeys conservation, meaning nothing is created/destroyed over the course of the reaction.
2. Its coefficients tell us molar ratios of each species. For example, 4 moles of Al requires 3 moles of O₂ to produce exactly 2 moles of Al₂O₃.

We can use the second point to solve this problem! 4 moles of aluminum react with every 3 moles of oxygen. However, 2.1 moles of oxygen is less than 3 moles given in the balanced reaction, so we'd predict that less than 4 moles of aluminum reacts, too.

More precisely:

2.1 mol oxygen * (4 mol aluminum/3 mol oxygen) = 2.8 mol aluminum.