THE MOLE
What’s a mole? It’s a number, like a dozen or a gross. Dozen, as you know,
means 12. Gross means 144. Mole means 6.02 × 10^23 . The number 6.02 × 10^23
is known as Avogadro’s number.
We said earlier that atomic mass is measured in atomic mass units (amu); 1 amu
is 1/12 the mass of a carbon-12 atom, or a relative mass of 1 gram (g). Each 1 g
of a substance has 6.02 × 10^23 amu. Think about what that means. If, for any
element, we take a sample whose mass in grams is numerically equal to its
atomic weight in amu, the sample has 1 mole of atoms in it. If we take a
substance whose mass in grams is numerically equal to twice its atomic weight
in amu, we have 2 moles of atoms. If we take a substance whose mass in grams
is numerically equal to three times its atomic weight in amu, we have 3 moles of
atoms. It’s as simple as that.
For example, helium’s atomic weight is 4 amu, so 4 g of helium contain 1 mole
(6.02 × 10^23 ) of helium atoms, and 8 g of helium contain 2 moles of helium
atoms. Carbon’s atomic weight is 12 amu, so in 12 g of carbon there is 1 mole
(6.02 × 10^23 ) of carbon atoms, and in 36 g of carbon there are 3 moles (18.06 ×
1023 ) of carbon atoms. Got it?
How many moles of oxygen molecules are in 64 g of oxygen gas? Remember
that oxygen is diatomic. Each O 2 molecule has a mass of roughly 2(16), or 32
amu, so 1 mole of O 2 molecules would have a mass of 32 g. Thus, in 64 g of
oxygen gas, there are 2 moles of oxygen molecules. Now how many moles of
oxygen atoms would be present in this 64 g sample? Each O 2 molecule is made
up of 2 oxygen atoms, so if the sample contains 2 moles of oxygen molecules, it
contains 4 moles of oxygen atoms.
Converting Mass Composition to Empirical Formula
Now that you know what an empirical formula is, we’ll tell you how to figure
out an empirical formula from the percent composition of a molecule. For
example, the test writers might tell you that some unknown substance is made up
of approximately 75% mercury and 25% chlorine, and they might ask you to