Concentration of Solutions

Concentration tells you how crowded a solution is — how much solute is packed into a given volume. It can be measured in moles, mass or parts per million, and switching between them is a core exam skill.

What concentration means

A solution is a solute dissolved in a solvent. Concentration measures how much solute sits in each unit of volume.

• A concentrated solution has a lot of solute per unit volume.
• A dilute solution has only a little.

Molar concentration (mol dm⁻³)

The most common measure is moles of solute per dm³ of solution. It’s often written in square brackets, e.g. [NaCl] = 0.25 mol dm⁻³.

A formula triangle links moles, concentration and volume:

Mass concentration (g dm⁻³)

Sometimes it’s easier to use mass per unit volume:

Convert between the two using the molar mass:

• mol dm^-3 → g dm^-3: multiply by the molar mass.
• g dm^-3 → mol dm^-3: divide by the molar mass.

Parts per million (ppm)

For very low concentrations — like a pollutant in water or air — we use parts per million (ppm).

• 1 ppm = 1 mg of solute dissolved in 1 dm³ of water.
• Since 1 dm³ of water weighs about 1 kg, that’s 1 mg in 1 kg — i.e. 1 part in 10⁶, or 1 in a million.