The Mole & the Avogadro Constant (AQA A-Level Chemistry): Revision Notes

1.3.1 The Mole & the Avogadro Constant

The mole is a fundamental unit in chemistry that allows scientists to count particles (atoms, molecules, ions, etc.) in a substance. It is an essential concept for understanding and performing chemical calculations, especially when dealing with large numbers of particles.

The Avogadro Constant

The Avogadro constant (L) is the number of particles (atoms, molecules, ions, etc.) contained in one mole of any substance.

The value of the Avogadro constant is approximately:

$$L = 6.02 \times 10^{23} \, \text{particles/mol}$$

This means that 1 mole of any substance contains 6.02 × 10²³ particles.

Application

The Avogadro constant is used to convert between the number of particles and the amount of substance in moles.

For example:

• 1 mole of carbon atoms contains 6.02 × 10²³ carbon atoms.
• 1 mole of water molecules contains 6.02 × 10²³ water molecules.

The Mole and Its Use in Chemical Calculations

The mole can be applied to electrons, atoms, molecules, ions, formulas, and equations. It serves as a bridge between the macroscopic and microscopic world, connecting the mass of a substance to the number of particles it contains.

1. Using Mass, $M_r$, and Amount in Moles

To calculate the amount of substance (in moles) from the mass of a substance, use the formula:

$$\text{Moles} = \frac{\text{Mass}}{M_r}$$

Where:

• Mass is the mass of the substance in grammes.
• $M_r$ is the relative molecular mass (or relative formula mass for ionic compounds).

2. Using Concentration, Volume, and Amount of Substance in Solution

To calculate the moles of a substance in a solution, use:

$$\text{Moles} = \text{Concentration (mol/dm³)} \times \text{Volume (dm³)}$$

This is useful for titrations and other reactions involving solutions.

Example Calculations