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Molar ConcentrationOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is concentration?

Concentration describes how much solute (dissolved substance) is present in a given volume of solution.

In chemistry, concentration is most often measured in moles per cubic decimetre (mol/dm³), sometimes written as mol dm⁻³.

A solution with a high concentration has many moles of solute dissolved in each dm³ of solution; a dilute solution has few.

Key termsconcentrationsolutesolution

Section 2

How do you calculate concentration in mol/dm³?

Use the equation:

Concentration (mol/dm³) = number of moles ÷ volume of solution (dm³)

This can be rearranged to find either of the other two quantities:

  • Number of moles = concentration × volume
  • Volume (dm³) = number of moles ÷ concentration
Example

0.5 mol of sodium hydroxide is dissolved in 2 dm³ of water. Concentration = 0.5 ÷ 2 = 0.25 mol/dm³.

Section 3

How do you convert between cm³ and dm³?

Volumes of solution are often measured in cm³ in the laboratory (e.g. using a burette or measuring cylinder), but the concentration equation needs the volume in dm³.

To convert cm³ to dm³, divide by 1000. To convert dm³ to cm³, multiply by 1000.

Common mistake

Forgetting to convert cm³ to dm³ is one of the most common errors in concentration calculations — always check the units in the question before substituting into the equation.

Section 4

How do you calculate the number of moles from concentration and volume?

Rearranging the concentration equation gives:

Number of moles = concentration (mol/dm³) × volume (dm³)

This is used constantly in titration calculations to work out how many moles of acid or alkali have reacted.

Example

25 cm³ of a solution with concentration 0.1 mol/dm³ is used. Volume in dm³ = 25 ÷ 1000 = 0.025 dm³. Moles = 0.1 × 0.025 = 0.0025 mol.

Must Know

  • Concentration (mol/dm³) = number of moles ÷ volume of solution (dm³)
  • Number of moles = concentration × volume (dm³)
  • Volume (dm³) = number of moles ÷ concentration
  • 1 dm³ = 1000 cm³ — always convert volumes given in cm³ before calculating
  • Higher concentration means more moles of solute per dm³ of solution
  • These equations are essential for titration calculations

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