Counting atoms inside a molecule
So far we have been counting molecules. But a molecule holds several atoms, so when atoms are asked for there is one more step.
Problem
For 5 moles of carbon dioxide molecules, find:
- the number of molecules
- the total number of atoms
- the number of oxygen atoms
Solution — number of molecules
molecules in 1 mol of CO2 = 6.022 × 1023
molecules in 5 mol of CO2 = 6.022 × 1023 × 5
= 3.011 × 1024
Solution — total number of atoms
total atoms in one CO2 molecule = 3
total number of atoms = 3.011 × 1024 × 3
= 9.033 × 1024
The 3 comes straight from the formula: one carbon and two oxygens.
Solution — number of oxygen atoms
oxygen atoms in one CO2 molecule = 2
number of O atoms = 3.011 × 1024 × 2
= 6.022 × 1024
A little more practice
How many moles of oxygen atoms are contained in one mole of each of these compounds? The answer is the oxygen subscript in the formula.
| Compound | Moles of O atoms |
|---|---|
| Al2O3 | 3 |
| CO2 | 2 |
| Cl2O7 | 7 |
| CH3COOH | 2 |
| Ba3(PO4)2 | 8 |
Brackets multiply
Ba3(PO4)2 has 8 oxygen atoms, not 4. The 2 outside the bracket acts on everything inside it: 4 × 2 = 8. In CH3COOH the O is written twice separately, which likewise makes 2.