### GCSE Chemistry

Quantitative Chemistry
Chemical Changes
Energy Changes
Organic Chemistry
Chemical Analysis
Earth and Atmospheric Science
Using Resources

We use moles (mol) to measure the amount of a substance. Specifically, one mole of a substance contains the same number of particles as one mole of another substance.

The Avogadro constant is the number of particles in one mole of a substance. These particles can be atoms, molecules, ions or electrons. The Avogadro constant is written in standard form as 6.022 × 10²³. But, this value is equal to 602,000,000,000,000,000,000,000.

• 1 mol of an element contains 6.022 × 10²³ atoms of that element
• 1 mol of a molecule contains 6.022 × 10²³ molecules

The mass of 6.022 × 10²³ particles of a substance is the same as the substance’s relative atomic or relative formula mass in grams.

For example, since the relative atomic mass of carbon is 12, 1 mol of carbon will weigh exactly 12 grams. In other words, 1 mol is also the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12.

• The mass of 1 mol of carbon dioxide (CO2) is (12 + 16 + 16) = 44 g

Particles in a substance can be calculated using both the Avogadro constant and the amount of substance in moles.

## Example

Calculate the number of atoms in 1 mol of sodium oxide (Na2O)

1 mol of Na2O is equal to 6.022 × 10²³ molecules of Na2O

As there are 3 atoms in Na2O:

The number of atoms = 6.022 × 10²³ × 3

= 1.807 × 10²⁴ atoms

## Example

Calculate the number of carbon dioxide (CO2) molecules in 0.8 mol of carbon dioxide.

One mole of CO2 contains 6.022 × 10²³ molecules of CO2.

Number of CO2 molecules = 6.022 × 10²³ × 0.8

= 4.818 × 10²³

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