All molecules contain more than one atom.
For example, in a carbon dioxide molecule (CO₂), there are 3 atoms – 1 carbon atom and 2 oxygen atoms.
Therefore, in 5 carbon dioxide molecules, there will be 15 atoms – 5 carbon atom and 10 oxygen atoms.
Similarly, in 1 mole carbon dioxide molecules, there will be 3 mole atoms – 1 mole carbon atoms and 2 mole oxygen atoms, and in 3 mole carbon dioxide molecules, there will be 9 mole atoms – 3 mole carbon atoms and 6 mole oxygen atoms, so on and so forth.

Example:

How many hydrogen molecules can be made with 20 mol of hydrogen atoms?
How many hydrogen atoms are there in 20 mol of hydrogen molecules?

Answer:
a.
Each hydrogen molecules (H₂) consist of 2 hydrogen atoms.
Therefore, 20 mol of hydrogen atoms can make 10 mole of hydrogen molecules.

b. Number of mole of hydrogen atoms in 20 mole of hydrogen molecules
= 2 x 20 mole = 40 mole.

Example:

Find the number of carbon atom in 0.75 mol of carbon dioxide.
Find the number of oxygen atom in 0.75 mol of carbon dioxide.

Answer:
a.
Each carbon dioxide molecules (CO₂) consist of 1 carbon atom and 2 oxygen molecules.
Therefore, 0.75 mole of carbon dioxide contain 0.75 mole carbon atoms.

b.
Number of mole of oxygen atoms in 0.75 mole of carbon dioxide
= 2 x 0.75 mole = 1.50 moles.

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