Colligative Properties of Solutions
Colligative properties are physical properties of a solution that depend only on the number of solute particles, not on the type of solute.
This means: Different substances can produce the same colligative effect if they have the same number of particles in solution.
Types of Colligative Properties
There are four main colligative properties:
1? Vapor Pressure Lowering
Explanation
When a non-volatile solute is added to a solvent, the vapor pressure of the solution decreases compared to the pure solvent.
Example:
Pure water evaporates faster than sugar solution.
Formula
Where:
P = lowering of vapor pressure
= vapor pressure of the pure solvent
= mole fraction of solute
2? Boiling Point Elevation
Explanation
A solution boils at a higher temperature than the pure solvent.
Example:
Pure water boils at 100C
Salt solution boils above 100C
Formula
Where:
Tb = boiling point elevation
= Vant Hoff factor
= boiling point elevation constant
= molality (mol solute/kg solvent)
3? Freezing Point Depression
Explanation
A solution freezes at a lower temperature than the pure solvent.
Example:
Pure water freezes at 0C
Salt solution freezes below 0C
Formula
Where:
Tf = freezing point depression
= freezing point depression constant
4? Osmotic Pressure
Explanation
Osmotic pressure is the minimum pressure required to stop osmosis through a semipermeable membrane.
Example:
Red blood cells shrink or burst due to osmotic pressure differences.
Formula
Where:
= osmotic pressure
M = molarity
R = gas constant
T = temperature (Kelvin)
Vant Hoff Factor (i)
The Vant Hoff factor represents the number of particles produced when a solute dissolves in a solution.
Substance
i
Glucose (CHO)
1
NaCl
2
CaCl
3
Non-electrolytes i = 1
Strong electrolytes i > 1
Summary
Colligative properties depend on the number of solute particles
They do not depend on the chemical nature of the solute
There are four colligative properties
More particles greater colligative effect
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