Define mass percentage, mole fraction, molarity and molality. Which of these depend on temperature, and why? Calculate the molality of a 10% (w/w) solution of glucose in water (M of glucose = 180 g mol⁻¹).

1. 1 · Define each term in one line.
   • Mass percentage = (mass of component ÷ mass of solution) × 100
   • Mole fraction = moles of a component ÷ total moles
   • Molarity = moles of solute per litre of solution
   • Molality = moles of solute per kilogram of solvent
2. 2 · Say which depend on temperature. Molarity (and volume percentage) change, because the volume of a solution changes with temperature. Mass percentage, mole fraction and molality do not, because mass does not change.
3. 3 · Set up the numerical. 10% (w/w) means 10 g of glucose in 100 g of solution, so the water is 90 g = 0.090 kg.
4. 4 · Find the moles of solute. n = 10 g ÷ 180 g mol⁻¹ = 0.0556 mol.
5. 5 · Calculate and box the answer. m = 0.0556 mol ÷ 0.090 kg = 0.617 mol kg⁻¹.
6. 6 · Link to real life. A 5% dextrose or 0.9% saline drip in a hospital is a concentration statement on the bottle.
7. 7 · Close in one line. So for work at changing temperatures, use molality or mole fraction, not molarity.

Scoring tip: Write the unit with every number: mol L⁻¹ for molarity and mol kg⁻¹ for molality. Convert grams of solvent to kilograms before dividing.