Chemistry: Converting Between Moles, Molecules, and Grams

Converting Between Moles, Molecules, and Grams

Once you know how to find molar mass, you can start to convert between moles, grams, and molecules of a substance. To do so, use the following figure:

Figure 11.2By using this map as a guide, you can easily learn to convert between grams, molecules, and moles of chemical compounds.

The method you'll use to solve this problem is the factor-label method. We'll see how to use the factor label method using the following example:

Example: Convert 63 grams of ammonia to molecules.

Solution: To answer this problem, do the following steps:

1. Write down the number you're trying to convert.
   63 g NH₃
2. Write a multiplication sign after the unit you're trying to convert, followed by a straight, horizontal line.
   63 g NH₃ × ________________
3. In the space below the line, write the unit of the number you're trying to convert.
   63 g NH₃ × ^{________________}⁄ g NH₃
4. Take a look at the map in the preceding figure. Find your starting point (in this case, "g") and move one box over toward your destination. Write the unit that you find in the next box (in this case, "mol") on top of the line.
   63 g NH₃ × ^{mol NH₃}⁄_{g NH3}
5. Write the conversion factor in front of the units you wrote above and below the line. This conversion factor can be found on the line between the starting and destination boxes in the figure (in this case, it's the molar mass of NH₃).
   ^{63 g NH₃ × 1 mol NH₃}⁄_{17 g NH3}
6. Multiply these numbers together, making sure to cancel out any appropriate units.
   63 g NH₃ × ^{1 mol NH₃}⁄_{17 g NH3} = 3.7 mol NH₃
7. If you haven't yet finished the problem, use the output from your first calculation to start a second calculation using steps 1-6.

In this case, we found that we have 3.7 moles of NH₃. However, we're trying to find the number of molecules of ammonia, not the number of moles. As a result, we need to go through the preceding six steps to find molecules. When this calculation is set up, it should look like this:

• 3.7 mol NH₃ × ^{6.02 × 1023 molecules NH₃}⁄_{1 mol NH3 molecules NH3} = 2.2 × 10²⁴ molecules

Our answer, then, is 2.2 × 10²⁴ molecules NH₃.