Molar Ratios

Return to Stoichiometry Menu


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 H2 + O2 ---> 2 H2O

What is the molar ratio between H2 and O2?

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 O2 and H2O?

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 H2 and H2O?

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 O3 ---> 3 O2

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

What is the molar ratio between O3 and O2?

What is the molar ratio between O2 and O3?

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!


Practice Problems

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

1) N2 + 3 H2 ---> 2 NH3

Write the molar ratios for N2 to H2 and NH3 to H2.

2) 2 SO2 + O2 ---> 2 SO3

Write the molar ratios for O2 to SO3 and O2 to SO2.

3) PCl3 + Cl2 ---> PCl5

Write the molar ratios for PCl3 to Cl2 and PCl3 to PCl5.

4) 4 NH3 + 3 O2 ---> 2 N2 + 6 H2O

Write the molar ratios for NH3 to N2 and H2O to O2.

5) Fe2O3 + 3 CO ---> 2 Fe + 3 CO2

Write the molar ratios for CO to CO2 and Fe to CO.


Answers

Return to Stoichiometry Menu