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:

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

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

Answer: two to one. So this ratio in fractional form is:

The ChemTeam recommends you explicitly write a one 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.

Here's another example equation:

2 O_{3}---> 3 O_{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 practice problems, 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, do the problems first!

Following each equation are two requests for molar ratios from the equation.

1) N_{2} + 3 H_{2} ---> 2 NH_{3}

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

2) 2 SO_{2} + O_{2} ---> 2 SO_{3}

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

3) PCl_{3} + Cl_{2} ---> PCl_{5}

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

4) 4 NH_{3} + 3 O_{2} ---> 2 N_{2} + 6 H_{2}O

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

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.