Concentration of Solutions — AQA GCSE Chemistry

The concentration of a solution tells you how much solute is dissolved in a given volume.

Calculating concentration in g/dm³

$$\text{concentration (g/dm}^3) = \frac{\text{mass of solute (g)}}{\text{volume (dm}^3)}$$

Remember 1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000 to get dm³.

• The more solute dissolved in a given volume, the more concentrated the solution.
• The larger the volume for the same mass, the more dilute the solution.

Worked example

Dissolve 20 g of salt in 250 cm³ of water.

• Volume = 250 ÷ 1000 = 0.25 dm³.
• Concentration = 20 ÷ 0.25 = 80 g/dm³.

Concentration in mol/dm³ (Higher Tier)

$$\text{concentration (mol/dm}^3) = \frac{\text{moles of solute}}{\text{volume (dm}^3)}$$

To convert between g/dm³ and mol/dm³, use the molar mass: divide by Mr to go from g/dm³ to mol/dm³.

Exam tips

• Learn concentration = mass ÷ volume (in dm³).
• Always convert cm³ to dm³ (÷ 1000).
• More solute or a smaller volume → more concentrated.
• For Higher Tier, be able to use mol/dm³ and convert using Mr.