Chemistry / Unit 10: Solutions, Acids and Bases
10.3 Colligative Properties
Notes
Colligative properties depend on the number of dissolved particles, not their identity.
- Dissolving a solute lowers the freezing point and raises the boiling point of a solvent. It also lowers vapor pressure.
- Molality $\displaystyle m = \frac{\text{mol solute}}{\text{kg solvent}}$.
- $\Delta T_f = i K_f m$ and $\Delta T_b = i K_b m$. For water, $K_f = 1.86$ °C/m and $K_b = 0.512$ °C/m.
- The van't Hoff factor $i$ is the number of particles per formula: sugar $i = 1$, NaCl $i = 2$, $\text{CaCl}_2$ $i = 3$.
- Examples: road salt melts ice; antifreeze keeps car engines from freezing and boiling over.
Practice
What is the freezing point of a 1.00 m NaCl solution in water? ($K_f = 1.86$ °C/m)
NaCl gives 2 ions, so $\Delta T_f = (2)(1.86)(1.00) = 3.72$ °C. The freezing point is $0 - 3.72 = -3.72$ °C.
Which solution has the lowest freezing point?
$\text{CaCl}_2$ makes 3 ions per formula unit, the most particles.
What is the boiling point of a 0.500 m glucose solution in water? ($K_b = 0.512$ °C/m)
Glucose does not split, so $i = 1$. $\Delta T_b = (1)(0.512)(0.500) = 0.256$ °C, so $100 + 0.256 = 100.26$ °C.
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