Chemistry: Simple Stoichiometry Calculations

Simple Stoichiometry Calculations

Now that you've done your first stoichiometric calculation using the ice water, it's time to move on to the sorts of questions that chemistry teachers like to ask. Here's one now:

Example: Using the equation 2 H_2(g) + O_2(g) ⇔ H₂O_(g), determine how many moles of water can be formed if six moles of oxygen are made to react with an excess of hydrogen.

Yikes! That doesn't look much like the ice water example, does it? Before we even attempt to solve this problem, let me give you a handy diagram that will solve all of your stoichiometric needs:

Figure 19.1This handy chart will help you with all mass-to-mass or mole-to-mole stoichiometry problems, like the one in our example.

To use the diagram, follow these steps:

1. Find the box that corresponds to the information that you were given in the problem. In our example, we were given "6 moles of oxygen," so the "Moles of Reactant" box is where we'll start. (If we were given "6 grams of oxygen" instead, we'd start at the box labeled "Moles of Reactant.")
2. Find the box that corresponds to the value that you're ultimately trying to find. Because our problem wants us to determine "how many moles of water can be formed," we want to end up at the box that corresponds to "Grams of Product."
3. Write down the number and unit that you were given in the problem on your paper.
   6 mol O₂
4. Write a multiplication sign after the number and unit you were given, followed by a straight, horizontal line.
   6 mol O₂ × _________________
5. Below the line, write the same unit that you were given. Include any chemical formulas.
   6 mol O₂ × ^{_________________}⁄_{mol O2}
6. The diagram I gave you is a map that tells you how to get to where you're going. From the box that corresponds to the information you were given in the problem, move one step toward your destination. Write the unit from this destination box above the line. In our example, since the next box says "Moles of Product," we'll write "mol H₂O" above the line.
   6 mol O₂ × ^{_____mol_H₂O_____}⁄mol O₂
7. From the line in between the boxes in the diagram, get the conversion factors for this calculation, and put them in front of the appropriate unit. In this example, our conversion factor is something called the "mole ratio," which consists of the ratio of moles of product to the ratio of moles of reactant. In our mole ratio step, we use the coefficients in the balanced equation to find what these numbers are:
   6 mol O₂ × ^{____2_mol_H₂O___}⁄1 mol O₂
8. Return to step 4 of this example, and continue the process until you have reached the destination box. In our case, the second box is the same as the destination box, so we don't need to continue. However, if we were trying to find "grams of water," we'd need to add another step to our conversion.
9. Once the calculation has been completely set up, solve the resulting equation to get the final answer. The unit in our answer will be the only one that doesn't cancel out—in this case, "mol H₂O."
   6 mol O₂ × ^{____2_mol_H₂O____}⁄_{1 mol O2} = 12 mol H₂O