The mole movie text

The mole is one of the central concepts in Stoichiometry. We will see it come up time and time again. A mole refers to a specific number of molecules: Avogadro's number or 6.02 x 10²³ molecules. A mole is analogous to a dozen in that both refer to a specific number of items.

A dozen eggs is 12 eggs. A dozen cars is 12 cars. A dozen water molecules is 12 molecules.

A mole of eggs would be 6.02x10²³ eggs, which is so many eggs it would never arise in everyday life. But a mole of water molecules, or 6.02x10²³ water molecules,is about the amount in a small test tube of water. Just as the dozen is a useful quantity for counting eggs, the mole is a useful quantity for counting molecules.

The mole allows us to connect the macroscopic world, where we all live, to the molecular scale, where chemistry occurs.

On the macroscopic scale, we think in terms of grams of a substance.

On the molecular scale, we think in terms of numbers of molecules, in moles. If a substance is in solution, we use molarity, which is moles per liter.

To connect between the macroscopic and molecular scale, we use the molecular weight.

The molecular weight is the number of grams per mole of a substance. This allows us to go back and forth between the amount of substance expressed in grams and the amount of substance expressed in number of molecules as in moles.

So where is this useful?

When we weigh out a certain number of grams of a substance, we are working at the macroscopic scale. If we then perform a chemical reaction, that reaction occurs at the molecular scale. For instance, in the reaction: [see figure below]

one molecule of oxygen reacts with two molecules of hydrogen to produce two molecules of water. So if we want to understand how much oxygen or hydrogen will be consumed in a reaction, or how much water will be produced, we need to think in terms of the number of molecules of oxygen, hydrogen and water. But in the laboratory, we must deal with grams of hydrogen, oxygen and water. The molecular weight is the key to going back and forth between the macroscopic scale, where we weigh things, and the microscopic scale, where molecules react with one another.

Another place where the mole is useful is when you know the number of molecules and need to find the weight. For example, chemists will often shine light on a sample and see how much light it absorbs. From this, they can obtain the number of molecules.

To simulate this in this course module, we will be using the virtual lab, which will show the moles or molarity directly.

This is a simpler experiment than the ones chemists actually do, because the virtual lab gives us the moles or molarity directly, while chemists would need to shine light on the sample and from that derive the moles or molarity. But the virtual lab is similar to the more complex experiments, since both give the number of molecules and we must use the molecular weight to relate this to the mass of the substance.