What effect do ions have?
The addition of ions creates significant changes in properties of solutions. Water molecules surround the ions and are somewhat tightly bound to them. Colligative properties are affected because the solvent properties are no longer the same as those in the pure solvent.
Electrolytes and Colligative Properties
Ionic compounds are electrolytes and dissociate into two or more ions as they dissolve. This must be taken into account when calculating the freezing and boiling points of electrolyte solutions. The sample problem below demonstrates how to calculate the freezing point and boiling point of a solution of calcium chloride. Calcium chloride dissociates into three ions according to the equation:
Sample Problem: Freezing and Boiling Point of an Electrolyte
Determine the freezing and boiling point of a solution prepared by dissolving 82.20 g of calcium chloride into 400.g of water.
Step 1: List the known quantities and plan the problem.
- mass CaCl2 = 82.20 g
- molar mass CaCl2 = 110.98 g/mol
- mass H2O = 400. g = 0.400 kg
Kf(H2O)=−1.86∘ C/m Kb(H2O)=0.512∘ C/m
- CaCl2 dissociates into 3 ions
Tf=? ∘C Tb=? ∘C
The moles of CaCl2 is first calculated, followed by the molality of the solution. The freezing and boiling points are then determined, including multiplying by 3 for the three ions.
Step 2: Solve.
Step 3: Think about your result.
- The effect of ionization on colligative properties is described.
- Why do ionic materials change the colligative properties of a solution?
- Would HCl be expected to alter colligative properties?
- Calcium carbonate is ionic, but insoluble in water. What effect would it have on the boiling point of water?