Mr. Groves Explains...

What is a Mole?

Chemists constantly deal with chemicals in two different ways...
the chemicals we can see and measure in the laboratory (like 16 g of CH4 or 32 g of O2, etc.) and the molecules we see in our heads that move and combine.
The "mole" provides a bridge between these two areas.

Molecular (or Atomic) Masses
CH4 H H2 O2
16 u 1 u 2 u 32 u

Through experiments we can find the relative mass of molecules. We know that one molecule of CH4 has a mass of 16 units (16 times heavier than a Hydrogen atom alone). We know that one molecule of oxygen, O2, has a mass of 32 units. Knowing the mass of just one molecule is not very helpful in the lab, however, because one molecule is just too small.

GRAM Molecular (or Atomic) Masses
CH4 H H2 O2
16 g 1 g 2 g 32 g
We COULD, however, measure out 16 GRAMS of CH4 or 32 GRAMS of O2. These amounts are called the molecular mass in grams or the "gram MOLEcular mass". One gram MOLEcular mass of CH4 is 16 g. One gram MOLEcular mass of O2 is 32 g.

The useful thing about these numbers is that when you measure out one gram MOLEcular mass of something (that is, its molecular mass or molar mass measured in grams) you have measured out the same number of molecules.

Numbers of Particles in Gram MOLEcular Masses
Substance Gram Molecular Mass Number of Particles
CH4 16 g 6.02 x 1023
O2 32 g 6.02 x 1023
CO2 44 g 6.02 x 1023
H2O 18 g 6.02 x 1023
16 g of CH4 contains the same number of molecules of CH4 as 32 g of O2 contains molecules of O2. The reason that one gram MOLEcular mass has twice the mass as the other is that each molecule of O2 has twice the mass of each CH4.

The analogy is the DOZEN. If you found the mass of a dozen pennies, the mass would be 24 g (if each penny has a mass of 2 g). If nickels have a mass of 4 grams each, then a dozen nickels would have a mass of 48 g. 24 g of pennies contains the same number of coins as 48 g of nickels because each nickel is twice as massive as each penny.

Masses of Money
Coin Mass of One Coin Dozen Mass Number of Coins
Nickel 4 g 48 g 12 nickels
Penny 2 g 24 g 12 pennies
Dime 1.5 g 18 g 12 dimes
Quarters 7 g 84 g 12 quarters
Back to CH4 and O2. The usefulness of this comes up when we look at experimental data. Say that we burn 16 g of CH4 in O2. We would find that 64 g of O2 is used up. 16 g CH4 + 64 g of O2 react (and form CO2 and H2O). 16 g CH4 is one gram MOLEcular mass of CH4. 64 g of O2 is TWO gram MOLEcular masses of O2... Since each gram MOLEcular mass of a substance contains the same number of molecules, then we know that 1 molecule of CH4 reacts with 2 molecules of O2. The gram MOLEcular mass is a bridge between the measurements we made in lab and the way that individual molecules (that we cannot see) react.

The only problem with gram MOLEcular mass is that the name is TOOOOO long. We shorten it down to "MOLE". So... 1 mole of CH4 is 16 g. 1 mole of O2 is 32 g. A dozen contains 12. Scientists have found that a mole contains 6.02 x 1023 molecules, that's 602 000 000 000 000 000 000 000 molecules!

A certain amount of a substance may be thought of in terms of its mass, its number of molecules, its number of moles, or even its volume (1 mole of gas at STP = 22.4 L). The mole provides conversion factors between these different ways of looking at a substance.