Mole Ratios — Definition & Overview

Mole Ratios

What is a mole ratio?

• The mole ratio is the relationship between the number of moles of each species in a balanced chemical equation. It tells us the proportions of each element or molecule in our chemical reaction.

• The mole ratio is important when converting from one molecule to another in a chemical reaction (typically used to convert from reactants to products).

In the image above, the coefficient of each species indicates the mole ratio. In this reaction, two sulfur (S) atoms react with three diatomic oxygen ($O_{2}$) molecules. They produce two sulfur trioxide ($SO_{3}$) molecules. So our mole ratio is 2:3:2.

• As a side note, sulfur trioxide is an interesting compound. It's a precursor to sulfuric acid. So, it gets produced on an industrial scale. If you were to draw the lewis structure, you would find that each oxygen is double bonded to the sulfur atom. So, sulfur is acting as an exception to the octet rule. It can do that because it has access to the d-orbitals. This also gives it the trigonal planar VSEPR geometry.

When do you use a mole ratio?

The mole ratio is used in stoichiometry problems. Stoichiometry studies the relationships between different quantities of reactants and products. You use the mole ratio to convert from one quantity in a reaction to another.

In future lessons, we'll learn how to use the mole ratio to...

• convert moles of reactant to moles of product.

• convert reactant mass to product mass.

• convert the volumes of reactants and products.

• calculate the limiting reagent.

• calculate theoretical, actual, and percent yields.

Example:

A scientist reacts 0.9873 mols of oxygen gas ($O_{2}$) with iron metal ($Fe$). How many moles of ferrous oxide ($FeO$) does he produce?

$2Fe$ + $O_{2}$ → $2FeO$

0.9873 mol $\cdot\left(\frac{2molFeO}{1molO2}\right)$ = 1.9746 mol FeO

• As you can see, the mole ratio between $O_{2}$ and $FeO$ was used to find the number of moles of $FeO$

(Video) Mole Ratio Practice Problems

by Tyler DeWitt

In this video, Tyler DeWitt explains mole ratios. He shows you two methods for each practice problem. The first he calls the recipe method. It treats chemical equations like a recipe. The second is the unit analysis method. It uses conversion factors. But, you'll get the same answer with both. Are you struggling with mole ratio problems? This video will give you the tools to solve any problem.