Colligative Properties Physical properties melting point and boiling

Colligative Properties Physical properties (melting point and boiling point) of solutions depend on the concentration of solute particles in solution

Freezing point depression • When a solute is dissolved in a liquid the freezing point of the solution will be lower ↓ than the pure liquid. The freezing point of water is less than 0 o. C if something is dissolved in it.

Boiling point Elevation • When a solute is dissolved in a liquid the boiling point of the solution will be higher ↑ than the pure liquid. The boiling point of water is more than 100 o. C if something is dissolved in it.

How much ΔT changes depends on Concentration of SOLUTE PARTICLES in sol’n 1 m electrolyte vs. 1 m non-electrolyte 1 m Na. Cl vs. 2 m Na. Cl 1 m Na. Cl vs. 2 m C 12 H 22 O 11

Which would result in the lowest freezing point / highest boiling point? • 1 m Na. Cl (aq) • 1 m C 12 H 22 O 11(aq) • 1 m C 2 H 6 O 2(aq) • 1 m Ca. Cl 2(aq)

Which would result in the lowest freezing point / highest boiling point? • 2 m Na. Cl (aq) • 1 m C 12 H 22 O 11(aq) • 1. 5 m C 2 H 6 O 2(aq) • 1 m Ca. Cl 2(aq)

Different solvents have different Kf & Kb

Formulae ∆Tb = Kb m ∆Tf = Kf m • Delta T = change in freezing point temperature (or boiling point temp) • Kb or Kf is Konstant • molality = mol of solute per Kg of solvent

Practice problem • I add 25. 0 g of Na. Cl to 100. 0 g of water. – What is the new boiling point? – What is the new freezing point? BP = 105. 65 o. C FP = -20. 3 o. C

Practice problem #2 • Benzene has a boiling point of 80. 1 o. C, when 6. 2 g of an unknown substance is added to 150. 0 g of benzene, its boiling point increases to 85. 2 o. C. What is the molar mass of the unknown substance? 20. 5 g/mol

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