Hello, Brooke,
To answer a question that involves the measure moles, we need the molar mass of the element or compound in question. The periodic table provides those answers. The atomic mass of each element can be found with its symbol, and is typically expressed as that element's "atomic mass unit," or AMU. There is a conversion that can be used to make this grams, but AMU makes the calculations easier and allows the assignment of mass values of 1 for protons and neutrons, and, for convenience, assigns "0" for the electrons. A single atom of Argon, for example, has 18 protons (which is fixed as the atomic number) and, on average, 22 neutrons. A neutral atom also has 18 electrons. The single atom I described has an AMU of 40. The periodic table differs by this precise amount due to the fact that some atoms of Ar have a few more or less neutrons. On average, in the Earth's crust, these "isotopes" cause the reported AMU to be slightly different than an exact number.
The fascinating utility of these AMU values is that they can be linked to Avogadro's number of 6.02 x 1023, which is a large number that is simply called a "mole." A mole is just a shorthand term for 6.02 x 1023. If I have a mole of eggs, I have 6.02 x 1023 eggs. For Argon, with an average AMU of 39.95, we can also restate that number as 39.95 grams/mole. In other words, the atomic weight of each element is also the molar mass of that element. If you have 39.95 grams of Ar, you'll have a mole of Ar atoms.
The same is true for all elements. 23 grams of sodium (23AMU) is also 23 g/mole Na, or one mole of sodium. Be careful with diatomic gases, since one such as oxygen (O) exists in nature as O2. This means that the AMU of 16 for each oxygen atom needs to be doubled to account that there are two oxygens per molecule. It's molar mass is thus 32g/mole. Molar masses of compounds are calculated by adding the AMUs of each atom in the compound. For H2O, the molar mass would be 1 + 1 + 16 = 18g/mole, for example.
Getting back to Ar, it's atomic mass is 39.95AMU and we can write that as 39.95 grams/mole. In other words, if we have 39.95 grams of Ar, we have one mole of Ar.
Since we only have 28.7g of Ar, we need to convert by dividing by the molar mass of argon:
28.7g Ar/(39.95g Ar/mole)
The grams cancel and mole moves to the top. So we have 0.718 moles of Ar. This makes sense - it takes 39.95 grams for one mole, but we only have 28.7 moles.
I hope this helps,
Bob
