The molar ratio will assume a place of central importance in solving stoichiometry problems. The sources for these ratios are the coefficients of a balanced equation. We will look at what a molar ratio is and then a brief word on how to recognize which ratio to use in a problem.

The ChemTeam's favorite sample equation is:

2H_{2}+ O_{2}---> 2H_{2}O

What is the molar ratio between H_{2} and O_{2}?

Answer: two-to-one. This ratio in fractional form is:

The ChemTeam recommends you always explicitly write the numeral one when it is in the denominator of the ratio.

What is the molar ratio between O_{2} and H_{2}O?

Answer: one-to-two. As a fraction, it is:

You knew that, of course. The ChemTeam apologizes if you felt like you were being treated like a dummy!

What is the molar ratio between H_{2} and H_{2}O?

Answer: two-to-two or:

As you well know, this reduces to one-to-one. However, the ChemTeam will use the unreduced fraction in the lessons to follow. The reason is this: in the classroom, the ChemTeam would often reduce the ratio to one-to-one and, sooner or later, someone would ask where the one-to-one ratio came from.

Here's another equation:

2O_{3}---> 3O_{2}

The exact molar ratio you would use depends on how the problem is worded.

What is the molar ratio between O_{3} and O_{2}?

What is the molar ratio between O_{2} and O_{3}?

The first one, of course, is:

And the second simply reverses the numbers:

Before looking at the following examples, an important point: the coefficients of a reaction only give the ratio in which substances react. They do not in any way tell you HOW MUCH is reacting. This point is elaborated upon in what the ChemTeam believes is the next logical unit from here. However, look at the examples first!

**Example #1:** N_{2} + 3H_{2} ---> 2NH_{3}

Write the molar ratios for N_{2} to H_{2} and NH_{3} to H_{2}.

**Example #2:** 2SO_{2} + O_{2} ---> 2SO_{3}

Write the molar ratios for O_{2} to SO_{3} and O_{2} to SO_{2}.

**Example #3:** PCl_{3} + Cl_{2} ---> PCl_{5}

Write the molar ratios for PCl_{3} to Cl_{2} and PCl_{3} to PCl_{5}.

**Example #4:** 4NH_{3} + 3O_{2} ---> 2N_{2} + 6H_{2}O

Write the molar ratios for NH_{3} to N_{2} and H_{2}O to O_{2}.

**Example #5:** Fe_{2}O_{3} + 3 CO ---> 2 Fe + 3 CO_{2}

Write the molar ratios for CO to CO_{2} and Fe to CO.