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Electrolytes and Colligative Properties

Describe effect of ionization on properties of solutions

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Electrolytes and Colligative Properties

The presence of ions in solutions affect the properties of that solution

Credit: Donnovan JPM
Source: http://commons.wikimedia.org/wiki/File:Ionic_solution_measure.jpg
License: CC BY-NC 3.0

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:

\begin{align*}\text{CaCl}_2 (s) \rightarrow \text{Ca}^{2+} (aq)+2\text{Cl}^-(aq)\end{align*}

The values of the freezing point depression and the boiling point elevation for a solution of CaCl2 will be three times greater than they would be for an equal molality of a nonelectrolyte.

 

 

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.

Known

  • mass CaCl2 = 82.20 g
  • molar mass CaCl2 = 110.98 g/mol
  • mass H2O = 400. g = 0.400 kg
  • \begin{align*}K_f(\text{H}_2\text{O})=-1.86^\circ \ \text{C}/ m\end{align*}
  • \begin{align*}K_b(\text{H}_2\text{O})=0.512^\circ \ \text{C}/ m \end{align*}
  • CaCl2 dissociates into 3 ions

Unknown

  • \begin{align*}T_f=? \ ^\circ \text{C}\end{align*}
  • \begin{align*}T_b=? \ ^\circ \text{C}\end{align*}

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.

\begin{align*}82.20 \text{ g CaCl}_2 \times \frac{1 \text{ mol CaCl}_2}{110.98 \text{ g CaCl}_2} &= 0.7407 \text{ mol CaCl}_2\\ \frac{0.7407 \text{ mol CaCl}_2}{0.400 \text{ kg H}_2\text{O}} &= 1.85 \ m \ \text{CaCl}_2\end{align*}

\begin{align*}\Delta T_f &= K_f \times m \times 3=-1.86^\circ \text{C}/m \times 1.85 \ m \times 3=-10.3^\circ \text{C} && T_f=-10.3^\circ \text{C}\\ \Delta T_b &= K_b \times m \times 3=0.512^\circ \text{C}/m \times 1.85 \ m \times 3=2.84^\circ \text{C} && T_b=102.84^\circ \text{C}\end{align*}

Step 3: Think about your result.

Since the normal boiling point of water is 100.00°C, the calculated result for \begin{align*}\Delta T_b\end{align*} must be added to 100.00 to find the new boiling point.

Summary

  • The effect of ionization on colligative properties is described.

Review

  1. Why do ionic materials change the colligative properties of a solution?
  2. Would HCl be expected to alter colligative properties?
  3. Calcium carbonate is ionic, but insoluble in water. What effect would it have on the boiling point of water?

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Image Attributions

  1. [1]^ Credit: Donnovan JPM; Source: http://commons.wikimedia.org/wiki/File:Ionic_solution_measure.jpg; License: CC BY-NC 3.0

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