E=Mz(Z
is the valency factor).
Some Important Relationships
Dilution Law: When we dilute a solution with solvent, the amount of solute remains constant, and we
can write:
M1 V1=M2 V2
and N1 V1=N2 V2
Molarity and Normality
Normality =z×
Molarity
Important:
The temperature has no effect on the mass per cent, ppm, mole fraction, or molality, however
temperature influences molarity and normality. This is because the volume is affected by temperature
whereas mass is not.
2. Vapour Pressure
2.1 Definition
At a given temperature, the vapour pressure of a liquid/solution is the pressure exerted by vapours in
equilibrium with the liquid/solution.
Vapour pressure ∝ escaping tendency
2.2. Vapour Pressure of Liquid Solutions and Raoult’s Law :
(Raoult’s law for volatile solutes)
Raoult's law states that the partial vapour pressure of each component in a solution of volatile liquids is
directly proportional to its mole fraction.
p1∝x1
and p1=p01x1
Component 2 is the same way.
p2=p20x2
The total pressure P(total)
over the solution phase in the container, according to Dalton's partial pressure law, is equal to the sum
of the partial pressures of the solution's components and is given as:
p(iotal) =p1+p2
We get by substituting the values of p1
and p2
p(total )=x1p01 and x2p02
=(1−x2)p01+x2p02
=p01+(p02−p01)x2
At constant temperature, the plot of vapour pressure and mole fraction of an ideal solution. The partial
pressures of the components are shown by the dashed lines I
and ∥.p1
and p2
are directly proportional to x1
and X2
, respectively, as can be seen from the plot. The total vapour pressure is represented in the figure by line
III.
Mole Fraction in The Vapour Phase
Using Dalton's rule of partial pressures, if y1
and y2
are the mole fractions of components 1 and 2 in the vapour phase, then:
p1=y1ptotal
p2=y2ptotal
Generally
pi=yiptotal
2.3. Vapour Pressures of Solutions of Solids in Liquids and Raoult’s Law
(Raoult’s law for non-volatile solutes)
When a non-volatile solute is added to a solvent to make a solution, the number of solvent molecules
leaving from the surface is reduced, lowering the vapour pressure.
The amount of non-volatile solute present in the solution, regardless of its composition, determines the
drop in solvent vapour pressure.
In its most general form, Raoult's law states that the partial vapour pressure of each volatile component
in a solution is directly proportional to its mole fraction for every solution.
p1∝x1
p1=x1p01=ptotal
If Raoult's law holds true for all concentrations, a solution's vapour pressure will vary linearly from zero
to the pure solvent's vapour pressure.
2.4. Ideal and Non-Ideal Solutions
Ideal Solutions:
An ideal solution is one in which each component follows Raoult's rule under all temperature and
concentration circumstances.
Properties of Ideal Solutions:
ΔHMIXING=0
ΔVMIXING=0
The attractive forces between the A-A and B-B molecules are approximately comparable to those
between the A-B molecules.
For example, benzene and toluene solution, n-hexane and n-heptane solution
Non – Ideal Solutions:
When a solution deviates from Raoult's law over a wide concentration range, it is referred to as a non-
ideal solution.
Solutions Showing Positive Deviation from Raoult’s Law:
SoluteSolute(B-B) and Solvent-Solvent(A-A) forces are weaker than Solvent-Solute(A-B) forces.
The vapour pressure is higher than the law predicts.
ΔHMIXING>0
ΔVMIXING>0
Ethanol with acetone, for example, or carbon disulphide and acetone.
