### StoichiometryMolar Ratio Examples

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

I will use it for the first three examples.

Example #1: What is the molar ratio between H2 and O2?

Solution:

The ratio is two-to-one. The 2 is the coefficient in front of the H2 and the 1 is the coefficient understood to be in front of the O2. Here is the ratio in fractional form:
 2 mol H2 ––––––– 1 mol O2
Make sure you also can recognize a ratio when it's written using a colon:
2:1

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

Note how the unit is included and the formula of the substance is included. This will help you to keep track of which units cancel when you start doing multi-step stoichiometry calculations.

Often the unit is not written, especially when a ratio is written using the colon format. This will create the potential for a problem when using the ratio values without the units. You run the risk of using the opposite ratio from the required ratio.

Be very careful to use the correct ratio!!

Example #2: What is the molar ratio between O2 and H2O?

Solution:

The ratio is one-to-two. The 1 is in front of the O2 and the 2 is in front of the H2O. As a fraction, it is:
 1 mol O2 –––––––– 2 mol H2O
The colon form is, of course:
1:2

Another way to see a ratio written is like this:

1 is to 2

Example #3: What is the molar ratio between H2 and H2O?

Solution:

The ratio is:
 2 mol H2 –––––––– 2 mol H2O
As you well know, this reduces to a one-to-one (or 1:1) ratio. For lessons that follow, the ChemTeam will mostly use the unreduced fraction. The reason is this: in the classroom, the ChemTeam would, from time to time, reduce the ratio to one-to-one and, sooner or later, someone would ask where the one-to-one ratio came from.

As the difficulty level of the problems goes up, the ChemTeam will just use the reduced ratio (assuming you have mastered the earlier materials, such as in this present tutorial). Also be aware, as you examine a solution to a problem by someone else, they may just use the reduced ratio without saying anything about it.

You have been warned!

Example #4: (a) What is the molar ratio between O3 and O2? (b) What is the molar ratio between O2 and O3?

2O3 ---> 3O2

Solution:

For (a), it is:

 2 mol O3 ––––––– 3 mol O2
And the answer to (b) simply reverses the numbers:
 3 mol O2 ––––––– 2 mol O3
As you can see, the exact molar ratio you would use depends on how the problem is worded.

However, a warning: people tend to play fast and loose with the molar ratio. The ChemTeam tends to put the first substance mentioned into the numerator. However, other people can be more haphazard. What they do is write a ratio without an explanation for how it got to be that way. What you have to do is figure out from context which number is associated with which substance. You do that by looking at the coefficients of the balanced equation.

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 remaining examples first!

By the way, still another way to show a ratio is this:

32

Done with units, it looks like this:

3 mol O22 mol O3

Make sure to carefully follow the units, especially which one isin the numerator and which one is in the denominator.

Also, notice I use the unit 'mol.' Don't use 'mole' because it is the name of the concept. The unit to be used in calculations is 'mol.'

Example #5: N2 + 3H2 ---> 2NH3

Write the molar ratios for (a) N2 to H2 and (b) NH3 to H2.

Solution:

The ratio for (a) is:
 1 mol N2 –––––––– 3 mol H2
and the ratio for (b) is:
 2 mol NH3 –––––––– 3 mol H2
Sometimes, a student will gather the mistaken impression that the molar ratio can only be constructed using the reactants of a given equation. The molar ratio can be constructed using any two compounds in the reaction, be they reactants or products.

Example #6: 2SO2 + O2 ---> 2SO3

Write the molar ratios for (a) O2 to SO3 and (b) SO2 to SO3.

Solution:

(a) is:
 1 mol O2 –––––––– 2 mol SO3
and (b) is:
 2 mol SO2 –––––––– 2 mol SO3

Example #7: PCl3 + Cl2 ---> PCl5

Write the molar ratios for (a) PCl3 to Cl2 and (b) PCl3 to PCl5.

Solution:

 1 mol PCl3 1 mol PCl3 (a) –––––––– (b) –––––––– 1 mol Cl2 1 mol PCl5

As the problems expand into multi-step solutions, there will be an interesting error students make when using a 1:1 ratio.

Example #8: 4NH3 + 3O2 ---> 2N2 + 6H2O

Write the molar ratios for (a) NH3 to N2 and (b) H2O to O2.

Solution:

 4 mol NH3 6 mol H2O (a) –––––––– (b) –––––––– 2 mol N2 3 mol O2

Note that both ratios can be reduced.

Eventually, ratios like the above will be used in multi-step calculations. You may use the unreduced ratio or the reduced ratio in the actual calculation. The ChemTeam's position is that it doesn't matter and so NEVER deducted points if the unreduced ratio was used. However, there are teachers who insist on the reduced ratio being used. Make sure you know what your teacher wants you to do.

Example #9: Fe2O3 + 3CO ---> 2Fe + 3CO2

Write the molar ratios for (a) CO to CO2 and (b) Fe to CO.

Solution:

 3 mol CO 2 mol Fe (a) –––––––– (b) –––––––– 3 mol CO2 3 mol CO

Notice that I stopped mentioning ratios like this: two-to-four. I have seldom seen then used, maybe two or three times. A word to the wise to be aware. Also, a reminder that you might see something like this:

two is to four

I'll do colon type molar ratios in the examples to follow. You need to do a bit of extra work to keep track of which value is associated with which chemical substance.

Example #10: In this equation:

C2H6O + 3O2 ---> 2CO2 + 3H2O

what is the mole ratio between O2 and H2O?

(a) 1:1; (b) 3:2; (c) 2:3; (d) 3:3

Solution:

From the coefficients of the equation, the mole ratio is 3:3. However, this reduces to a 1:1 ratio. That means that answer choice (a) would be considered by most teachers to be the correct answer.

Please note that using a 3:3 ratio in a calculation is equivalent to using a 1:1 ratio. The same answer is obtained using 3:3 as opposed to using 1:1.

The ChemTeam, when in the classroom (now retired), would use the unreduced ratio in calculations done for teaching purposes. This is because, invariably, if the reduced ratio is used, the question would be asked: "Where did the 1:1 ratio come from?"

Example #11: Given the balanced equation:

2SO2 + O2 ---> 2SO3

what is the mole ratio of O2 to SO3?

(a) 2:1; (b) 1:2; (c) 2:2; (d) 2:3

Solution:

The correct answer is (b) 1:2.

When I saw this problem online, 2:1 was the answer given, the reverse of the correct answer. 2:1 is the molar ratio of SO3 to O2.

Make sure to write the numbers of the ratio in the same order as used in the question. The first substance mentioned goes in the numerator, the second mentioned in the denominator.

Example #12: What is the molar ratio between copper(II) sulfate and water in the following compound?

CuSO4 5H2O

copper(II) sulfate pentahydrate

Solution:

Notice that this is not a chemical reaction, it is simply a chemical formula of a hydrate. Remember, hydrates have a fixed amount of water per mole of the non-water compound.

In this example, the molar ratio is 1:5. For every one mole of CuSO4, there are five moles of water.

Example #13: What is the molar ratio between the anhydrous compound and water for the following hydrates?

(a) CuCl2 2H2O
(b) MgSO4 H2O
(c) Cr2(SO4)3 18H2O

Solution:

(a) 1:2 ---> one mole of CuCl2 to two moles of water
(b) 1:1 ---> one mole of MgSO4 to one mole of water
(c) 1:18 ---> one mole of Cr2(SO4)3 to eighteen moles of water

Example #14: What is the molar ratio between potassium carbonate and water in the following compound?

K2CO3 1.5H2O ---> named potassium carbonate sesquihydrate

Solution:

The ratio is 1 to 1.5. In chemistry, it is common practice to state ratios as small whole numbers, so this ratio would be expressed as 2:3. Thus, this formula is often written as:

2K2CO3 3H2O

There is nothing whatsoever wrong with the 1 to 1.5 ratio. It's just that the common practice within the chemistry community is to use ratios expressed as small whole numbers.

Example #15: What is the molar ratio between calcium sulfate and water in the following compound?

CaSO4 0.5H2O ---> named calcium sulfate hemihydrate

Solution:

In small whole numbers, the ratio is 2:1 and the formula can be written thusly:

2CaSO4 H2O

Example #16: Which of the following statements about the mole ratio of a chemical equation is accurate?

(a) The mole ratio can describe the amount of product expected from a given amount of reactants.
(b) The mole ratio in a chemical equation can be determined using the coefficients in a balanced equation.
(c) The mole ratio can describe the ratio of reactants required to react.
(d) All of the above. <--- that's the answer