Answer: Moles of solute / (moles of solute + moles of solvent).
- A Moles of solute divided by the volume of solution in litres
- B Moles of solute / (moles of solute + moles of solvent)
- C Moles of solute divided by moles of solvent alone
- D Mass of solute divided by total mass of solution
Correct answer: B. Moles of solute / (moles of solute + moles of solvent)
Explanation: Mole fraction = n(solute) / (n(solute) + n(solvent)). It is dimensionless and sums to 1 for all components.
Adding a non-volatile solute lowers the solvent's vapour pressure at every temperature, which is the root cause behind all four colligative properties: it shifts the freezing point down and the boiling point up.
Concept context
Learn how substances dissolve and form mixtures. Covers concentration terms, colligative properties (boiling point elevation, freezing point depression, osmotic pressure), and ideal vs non-ideal solution behaviour.