Molar Ratios

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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:

2H2 + O2 ---> 2H2O

What is the molar ratio between H2 and O2?

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 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. 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:

2O3 ---> 3O2

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 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: N2 + 3H2 ---> 2NH3

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

Example #2: 2SO2 + O2 ---> 2SO3

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

Example #3: PCl3 + Cl2 ---> PCl5

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

Example #4: 4NH3 + 3O2 ---> 2N2 + 6H2O

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

Example #5: Fe2O3 + 3 CO ---> 2 Fe + 3 CO2

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

Answers to the five examples

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