Type of Solutions
Vapour Pressure:
Raoult’s Law:
“The partial vapour pressure of any component in the solution is directly proportional to its mole fraction”.
For a binary solution of two components A and B,
PA = XA
PB = XB
Where
P0A = vapour pressure of component A in pure state.
PA = vapour pressure of component A in the solution.
P0B = vapour pressure of component B in pure state.
PB = vapour pressure of component B in the solution
Limitations of Raoult’s Law
• Raoult’s law is applicable only to very dilute solutions.
• It is applicable to solutions containing non-volatile solute only.
• It is not applicable to solutes which dissociate or associate in a particular solution
Raoult’s Law in Combination with Dalton’s Law of Partial Pressure:
PT = XA P0A + XB P0B = P0B + (P0A -P0B) XA
Where
PT = Total Vapour Pressure of the Solution.
Ideal and Non-Ideal Solutions:
• Ideal Solution:
For Ideal Solution:
1. dHmixing = 0, i.e. no heat should be absorbed or evolved during mixing
2. dVmixing = 0, i.e. no expansion or contraction on mixing
Examples , Ethyl chloride and ethyl bromide, n–hexane and n–heptane , CCl4 and SiCl4
• Non-Ideal Solution:
?These solutions deviate from ideal behaviour and do not obey Raoult’s law over entire range of
composition.
For non ideal solutions,
1. dHmixing ≠ 0
2. dHmixing ≠ 0
Here we may have two cases
A) Positive Deviation:
1. PA > XA & PB > XB
2. dHmix > 0
3. dVmix > 0
Example: Cyclohexane and Ethanol
B) Negative Deviation:
1. PA > XA & PB > XB
2. dHmix < 0
3. dVmix < 0
Colligative Properties
Lowering of Vapour Pressure by a Non-Volatile Solute
Elevation of Boiling Point by a Non-Volatile Solute :
Depression of Freezing Point by a Non-Volatile Solute:
Osmosis and Osmotic Pressure:
• Osmosis: The phenomenon of the passage of pure solvent from a region of lower concentration
(of the solution) to a region of its higher concentration through a semi-permeable membrane.
• Osmotic Pressure: Excess pressure which must be applied to a solution in order to prevent flow of solvent into the solution through the semi-permeable membrane.
V = nRT
where
= Osmotic pressure
V = volume of solution
n = no. of moles of solute that is dissolved
R = Gas constant
T = Absolute temperature
Isotonic Solutions: A pair of solutions having same osomotic pressure is called isotonic solutions.
Abnormal Molecular Weight and Van't Hoff Factor:
Van't Hoff Factor:
Van't Hoff, in order to account for all abnormal cases introduced a factor i known as the Van't Hoff
factor, such that
Degree of Association:
Let a be the degree of association, then,
The number of unassociated moles = 1-α
The number of associated moles = α/n
Total number of effective moles = 1- α+ α /n
Obviously, i < 1
Degree of Dissociation
The fraction of the total number of molecules which dissociates in the solution, that is, breaks into simpler molecules or ions.
KCl ↔ K+ + Cl-
1-α α α
Thus, the total number of moles after dissociation = 1-α + α+α = 1+α
Hence, i = (1+α )/1
i = 1+α = 1+ (2–1)
In general, i = 1+ (n–1) α,
Where, n = number of particles ( ions) formed after dissociation
From the above formula, it is clear that i > 1
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