Chapter 18 Notes, part III
Colligative Properties
What are colligative properties?
Properties that are only affected by the
number of particles dissolved in a solvent.
Ionic vs. Covalent in Colligative Properties
As discussed before, when a salt dissolves in
water it dissociates.
This means there are more particles in a
solution of a salt in comparison to a solution
with a covalent compound (such as sugar).
How many particles do the following break into:
NaCl
CaCl2
C6H12O6
H3PO4
CO2
KNO3
N2O
Na2CO3
AlF3
C3H6O
Be3N2
SO3
Three Colligative Properties
Boiling Point Elevation
Freezing Point Depression
Vapor Pressure
Boiling Point Elevation
When a solute is added to a solution, the
boiling point of the liquid is increased.
It takes more kinetic energy to get the
molecules out of the liquid phase and into
the gas phase!
Why is this?
Freezing Point Depression
When a solute is added the freezing
point of a solvent is decreased.
Why does this occur?
Vapor Pressure
There is a decrease in vapor pressure
when a solute is added to a solvent.
This is because the solvent’s particles are
busy surrounding the solute, so less are
free to escape into the air.
Calculating Change in Boiling/Freezing Point
The change in boiling point can be found
using the formula:
DTb=Kb x m
For water,
Kb=0.512oC/m
The change in freezing point can be found
using the formula:
DTf=Kf x m
For water,
Kf=1.86oC/m
Practice Problem #1
What temperature does a salt solution boil at
if 95.5g of NaCl has been added to 1.5kg of
water?
Practice Problem #2
At what temperature does a solution freeze if
it is made by adding 52.2g of MgCl2 to
1127g of water?
Practice Problem #3
What is the molar mass of a sugar if when
235.8g are dissolved into 515g of water, the
boiling point is 101.3oC?
Practice Problem #4
What is the molar mass of a salt that breaks
into two parts, if when 122.5g are dissolved
into 495g of water, the boiling point is
103.1oC?