The Mole

Context

Once matter, elements, compounds, and molecules are understood, there needs to be a way to describe the number of atoms or the number of molecules present in any given situation. Knowing this will allow calculation of how many grams of a substance are needed for a given chemical reaction or how many grams of a substance will be produced in a chemical reaction.

The focus here is to understand what a mole is and be able to apply the concept in a variety of ways.

What is the mole?

The mole in chemistry is a unit of measure. It measures the number of objects. There are 6.02 x 10²³ objects in a mole.

Explanation

The dozen is another unit of measure for the number of objects. Everyone knows that one dozen apples means 12 apples. If someone says that there are 1.5 dozen eggs, it is understood that there are 18 eggs. Four dozen would be 48, etc.

It is the same with the mole. One mole of eggs would be 6.02 x 10²³ eggs. You better have a very large space to hold them all! It isn't very handy to talk about moles of eggs or apples. It also isn't very handy to talk about dozens of electrons. They are so small that a dozen electrons wouldn't be noticeable. Atoms are also very small. A dozen atoms isn't practical. There needs to be a "giga million" (my term for more than you can imagine) atoms just to be able to see anything. A better unit to measure the number of atoms is the mole. It turns out that a mole of atoms is a handy number of atoms when working in the laboratory.

Models

Dozen Models

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If a  •  is an egg, how many dozen eggs are in each of the following boxes?

• •    •  •  •• A

•    ••  • ••     •   •• B

•    •      •     • C

When you have decided on your answer, check it out.

If a  ¤  symbol represents six eggs, how many dozen eggs are in each of the following boxes? How many eggs are in each box?

¤ ¤    ¤  ¤  ¤¤ A

¤    ¤      ¤   ¤ B

When you have decided on your answer, check it out.

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Mole Models

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The same ideas can be applied to moles. Here are some for you to try.

If a  •  now represents 1 x 10²³ atoms, approximately how many moles of atoms are in each of the following boxes?

• •    •  •  •• A

•    ••  • ••     •   •• B

•    •      •     • C

When you have decided on your answer, check it out.

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Word Models

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Here are a some word problems for practice.

1. How many moles of water, H₂O, molecules are there if you have 8.2 x 10²³ H₂O molecules?
2. How many moles of sodium, Na, atoms are there if you have 3.0 X 10¹⁴ Na atoms?
3. How many atoms to you have if there are 1.2 moles of atoms?
4. How many carbon monoxide, CO, molecules do you have if there are .041 moles of CO molecules?

Here are the answers.

Thinking Questions

1. If there are the same number of moles of two different substances, will they take up the same amount of space?
2. Will ten moles of sodium and ten moles of lead have the same weight?
3. What is the difference between the number of atoms and the moles of atoms?