In the last chapter, we reviewed the process of balancing equationsand based the rules for balancing equations on the principle thatmatter is neither created nor destroyed in the course of a chemicalreaction. With this idea still in mind, let’s begin our discussion ofmoles and formula weights.
When you look at the periodic table, yousee that one of the pieces of data given for each element is its atomicweight. But what exactly is the atomic weight of a substance? It is themass of one mole of a substance. In turn, one
mole of a substance is equal to 6.02
1023 atoms or molecules of the substance (depending on what it is), and finally, the number 6.02
1023 is known as
Avogadro’s number. For example, carbon’s atomic weight is roughly 12 amu; this means that 6.02
1023 carbon atoms, in a pile, weigh 12 grams.
In order to find the
formula weightof a substance, you simply add up the atomic masses of all of the atomsin the molecular formula of a compound. But don’t forget to multiplythe atomic mass of each element by the subscript behind that element.Formula weights have the units amu, or atomic mass units; for example,the formula weight of water, H2O, is about 18 amu. (O = 16 amu plus 2 times H = 1 amu = 18 amu.) Similarly, the
molar mass of a molecule is the mass (in grams) of 1 mol of a substance; so the molar mass of H2O is also roughly 18.
Now try calculating some molar masses and formula weights on your own by filling in the following chart.
Example
| Substance | Molar mass | Number of moles | Mass in grams | Number of particles |
| Carbon dioxide, CO2 |
| 3.0 |
|
|
| Oxygen, O2 |
|
| 64.0 |
|
| Methane, CH4 |
| 0.279 |
|
|
| Nitrogen, N2 |
|
|
| 9.50 1025 |
Explanation
Three significant digits were used throughout,with the exception of molar masses, where two decimal places were used.But don’t stress over significant figures for this test: it’s multiplechoice, and the answers will never be that precise. Here’s the table,filled in.
| Substance | Molar mass | Number of moles | Mass in grams | Number of particles |
| Carbon dioxide, CO2 | 44.01 | 3.00 | 132 | 1.81 1024 |
| Oxygen, O2 | 32.00 | 2.00 | 64.0 | 1.20 1024 |
| Methane, CH4 | 16.05 | 0.279 | 4.48 | 1.68 1023 |
| Nitrogen, N2 | 28.02 | 158.00 | 4430 | 9.50 1025 |
Now that you’ve had some practice figuringout molecular weights, let’s talk about how you’ll be expected to usethem, and other stoichiometric tools, on the exam. For example, youwill almost certainly be asked to find the percent composition of acompound, so let’s talk about that first.
