Colligative Properties
Colligative Properties
Colligative Properties
REG # 2020-AG-8908
OR
4.Osmotic Pressure
On adding solute to solvent :
● VAPOUR PRESSURE (Decreases)
● MELTING POINT (Decreases)
● BOILING POINT (Increases)
● Osmosis (Osmotic Pressure)
Relative vapor pressure lowering is equal to the mole fraction of the solute
Example
The vapour pressure of dilute aqueous solution of glucose is 750 mm of Hg at 373K. Calculate mole fraction of
solute?
Solution:
p =760 mm
ps =750 mm
760-750/750 =
= 10/750
= 0.013 Ans
APPLICATION OF VAPOUR PRESSURE IN
DAILY LIFE
Adding solute lowers the vapour pressure during cooking purposes.
BOILING POINT ELEVATION
Boiling point is the temperature at which vapour pressure of a liquid becomes equal
to externa; atmospheric pressure (760mm Hg) .
When we add solute i.e sugar vapour pressure of solvent decreases at a constant
temperature.
This means solution (sugar + water) boils at higher temperature (102.8°C) than
temperature at which solvent ( water ) boils(100.0°C).
This rise due to addition of solute molecules is called as boiling point elevation.
This elevation in boiling point is due to non-volatile solute and solvent
interactions.
is the molal boiling point elevation constant and is for the solvent.
Solution:
Δt = Kb m
∆t= 3.91 °C
FREEZING POINT DEPRESSION
where ,
is the freezing point depression,
2. Road salting : Lowering the freezing point allows the street ice to melt at lower temperatures.
Example ( use of equation )
What is the freezing point depression when 62.2 g of toluene (C7H8) is dissolved in 481 g of naphthalene?
The freezing point constant for naphthalene is 7.00 °C/m.
Solution:
= 9.82 °C
that's not the new freezing point, that's the amount the freezing point is depressed
Only solvent molecules are able to pass through the membrane solute molecules
cannot pass across through it.
OR
the pressure that would have to be applied to a pure solvent to prevent it from
passing into a given solution by osmosis.
Factors affecting osmotic pressure
1.Solute concentration :
The osmotic pressure of a dilute solution at constant temperature is directly proportional to its concentration.
2.Temperature :
C is molarity
Osmotic pressure is pressure required to stop osmosis.
Osmosis and Blood cells
Osmotic pressure is important factor affecting cells.
Rearranging the osmotic pressure equation, the following equation can be obtained:
π = iCRT ; C = π/(iRT)
Here, the value of i is 2 (since KCl dissociates into two ions). Therefore, the molarity of KCl
is:
C = (50 atm)/(2)*(0.0821 atm.L.mol-1.K-1)*(300K)
C = 50/49.26 M = 1.015 M
Therefore, the molar concentration of potassium chloride in the solution is 1.015 M.
Sources :
1. Martin’s Physical pharmacy and pharmaceutical sciences by
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