Types of solutions and concentration

The essentials

A solution is a homogeneous mixture. The component present in the larger amount is usually called the solvent, and the others are solutes. Solutions are classed by the physical state of the solvent: gaseous (such as air), liquid (such as sugar in water, or oxygen dissolved in water) and solid (such as alloys, or hydrogen in palladium). A binary solution has two components.

Key points

• Mass percentage = (mass of component ÷ total mass of solution) × 100.
• Volume percentage = (volume of component ÷ total volume) × 100.
• Parts per million (ppm) = (parts of component ÷ total parts) × 10⁶. Used for very dilute solutions, such as pollutants in water.
• Mole fraction x = moles of component ÷ total moles. In a binary solution, x₁ + x₂ = 1.
• Molarity (M) = moles of solute ÷ volume of solution in litres (mol L⁻¹).
• Molality (m) = moles of solute ÷ mass of solvent in kilograms (mol kg⁻¹).
• Mass %, ppm (by mass), mole fraction and molality do not depend on temperature because they involve only masses. Molarity changes with temperature because volume changes.

Exam tips

• Molarity uses the volume of the whole solution, while molality uses the mass of the solvent only.
• Convert millilitres to litres and grams to kilograms before substituting.
• Example: 4 g NaOH (molar mass 40 g mol⁻¹) in 250 mL of solution is 0.1 mol ÷ 0.25 L = 0.4 M.

Common mistakes

• Using the mass of the solution instead of the mass of the solvent for molality.
• Forgetting that mole fractions of all components must add up to 1.