Title: Mole Ratio Calcs
Course: Stoichiometry

Calculations Involving Moles

Chemists aren't usually concerned with how many hot dogs they can buy on a given day. They would likely be concerned about reactions that they are going to perform.

Consider the following reaction:

2 H_2 + O_2 ==> 2 H_2O

Suppose a chemist wanted to perform this reaction to produce water. There would be several things the chemist would need to consider. Among the considerations:
1. How much H_2 is needed
2. How much H_2O will be produced
3. Safety concerns: This reaction gives off a lot of energy!

For this particular problem, a chemist needs to produce 40.0 mol H_2O. How many mol H_2 must the chemist react?

Step 1: Write what you are given.

In this problem, the given is 40.0 mol H_2O.

Step 2: Pick the correct mole ratio conversion factor.

The following are all of the possible mole ratios:

You only need one of them, so which one do you pick? Note the following:
1. The given in the problem is "40 mol H_2O"
2. The unknown in the problem is "mol of H_2"

Therefore, you should choose a mole ratio that has BOTH "H_2" and "H_2O" in it. Since mol H_2O is the unit on the top (40 mol H_2O), choose the ratio that has mol H_2O on the bottom.

Step 3: Cancel the units.
Cancel the same units that appear on the top and bottom.

Step 4: Do the math.
Remember to multiply by #'s on the top and divide by #'s on the bottom.

The answer should be reported as 40.0 mol H_2. The given in this problem (40.0 mol H_2O) had 3 sig figs, therefore the answer should have 3 sig figs as well.

Before clicking "NEXT" to go on to the next page, see if you can do the following problem on paper. Then click "NEXT" and it will be explained for you on the next page.

Using the reaction:

2 H_2 + O_2 ==> 2 H_2O

Determine how many mol O_2 are needed to react with 36 mol H_2.

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