13.1 Introduction to Colligative Properties, the van't Hoff factor, and Molality

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In this lesson you will learn:

-What a colligative property is and the four most common colligative properties

-What the van't Hoff factor is and how to identify it for a substance

-The definition of molality and how to calculate molality


What is a Colligative Property?

A colligative property is a property of a solution that changes proportionally as a solute is added, and this change generally occurs with any solute and is not dependent upon the solute's identity.  The key is that the solute is distinct from the solvent and disrupts the pattern of intermolecular forces normally present in the pure solvent, and the more solute particles the greater the disruption.

 

There are four commonly studied colligative properties:

  • Freezing Point Depression
  • Boiling Point Elevation
  • Vapor Pressure Depression
  • Osmotic Pressure

 

As you add increasing concentrations of solute to water its freezing point decreases, its boiling point increases, and the vapor pressure of water above the solution decreases.

 

Each of these are covered in more depth in the following lessons:

13.2 Calculations Involving Freezing Point Depression and Boiling Point Elevation

13.3 Vapor Pressure Depression and Raoult’s Law

13.4 Osmotic Pressure

What is the Van't Hoff Factor?

The van't Hoff Factor (i) has no units and is simply the number of ions a solute dissociates into.  Since the change in a colligative property is proportional to the number of solute particles present in solution, then a solute that dissociates into multiple ions will result in a larger change and the van't Hoff factor allows us to factor that in.  If a solute is a nonelectrolyte (includes most molecular compounds) and doesn't dissociate into ions it has a van't Hoff factor of 1.

 

The van't Hoff factors for some typical compounds are shown below:

 

CH3OH      i = 1 (nonelectrolyte)

 

C6H12O6   i = 1 (nonelectrolyte)

NaCl → Na+ + Cl-                       i = 2

van't hoff factor NaCl

CaCl2 → Ca2+ + 2Cl-                 i = 3

van't hoff factor CaCl2

Al(NO3)3 → Al3+ + 3NO3-         i = 4

For strong electrolytes having polyatomic ions do not make the mistake of trying to have the polyatomic ions dissociate into individual atoms.

van't hoff factor of Al(NO3)3

What is Molality?

Molality is a measure of concentration that is commonly used in calculations for some of the colligative properties.  It is similar to molarity but is dependent upon the number of kilograms of solvent rather than the volume of solution.

molality formula