1st Lecture on Solutions | Chemistry Part I | 12th Science by Rizwana Mohammad

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
2nd Lecture on Solutions
Chemistry Part I, 12th Science
By
Rizwana Mohammad

2. Colligative properties of nonelectrolyte solutions:
The physical properties of solutions that depend on the
number of solute particles in solutions and not on their
nature are called Colligative properties. These are
1. Vapour pressure lowering
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure

3. 1. Vapour pressure lowering:
When a liquid in a closed container is in equilibrium with its
vapour, the pressure exerted by the vapour on the liquid is its
vapour pressure.
When a non-volatile, non-ionizable solid is dissolved in a liquid
solvent, the vapour pressure of the solution is lower than that of
pure solvent.
if P1
0 = Vapour pressure of pure solvent
P1 = Vapour pressure of solvent above the solution,
therefore P1 < P1
0
ΔP = P1
0 - P1

4. Raoult’s law for solutions of non-volatile solutes:
A plot of P1 Vs x1 is a straight line.
Fig 1 : Variation of vapour pressure of solution with mole fraction solvent
For binary solution
x1 + x2 = 1
x1 = 1 – x2
P1 = P1
0 x1
= P1
0 (1 - x2)
= P1
0 – P1
0 x2
P1
0 – P1 = P1
0 x2
ΔP = P1
0 x2
The above equation shows ΔP depends on x2 i.e. on number of solute particles.
Thus, ΔP is a colligative property.

5. Relative lowering of vapour pressure:
The ratio of vapour pressure lowering of solvent divided by the
vapour pressure of pure solvent is called relative lowering of
vapour pressure.
Thus,
Relative vapour pressure lowering =
Δ𝑃
𝑃1
0 =
𝑃1
0
−𝑃1
𝑃1
0
Δ𝑃
𝑃1
0 = x2
𝑃1
0
−𝑃1
𝑃1
0 = x2

6. Molar mass of solute from vapour pressure lowering:
Δ𝑃
𝑃1
0 = 𝑥2
We know that 𝑥2 =
𝑛2
𝑛1
+𝑛2
n1 and n2 are the moles of solvent and solute respectively.
n1 >> n2
n1 + n2 ≈ n1
Therefore 𝑥2 =
𝑛2
𝑛1
Δ𝑃
𝑃1
0 =
𝑛2
𝑛1
…..1
n1 =
𝑤1
𝑀1
, n2 =
𝑤2
𝑀2
From eq. 1
Δ𝑃
𝑃1
0
=
𝑤2
𝑀2
𝑤1
𝑀1
𝑃1
0
−𝑃1
𝑃1
0 =
Δ𝑃
𝑃1
0 =
𝑊2
𝑀1
𝑀2
𝑊1

7. Boiling point elevation:
The boiling point of liquid is the temperature at which its
vapour pressure equals the applied pressure.
The boiling point of solution of non-volatile solute is higher
than that of pure solvent.
if Tb
0 = B.P. of pure solvent
Tb = B.P. of solution
Therefore, Tb > Tb
0
ΔTb = Tb - Tb
0
Boiling point elevation as a consequence of vapour
pressure lowering:
Fig 2 : Vapour pressure-
temperature of pure solvent
and solution.

8. Boiling point elevation and concentration of solute:
The boiling point elevation is directly proportional to the
molality of the solution.
Thus, ΔTb ∝ m
ΔTb = Kbm
m = molality of solution
Kb = boiling point elevation constant or molal elevation
constant or ebullioscopic constant.
If m = 1 , ΔTb = Kb
Kb is the boiling point elevation produced by 1 molal solution.
Unit of Kb = K kg mol-1

9. Molar mass of solute from boiling point elevation:
ΔTb = Kbm …1
m =
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑖𝑛 𝑘𝑔
=
𝑊2
𝑀2
𝑚𝑜𝑙
𝑊1
1000
𝑘𝑔
=
1000 𝑊2
𝑀2
𝑊1
𝑚𝑜𝑙 𝑘𝑔
− 1 …2
from eq. 1 & 2
ΔTb = Kb
1000 𝑊2
𝑀2
𝑊1
Hence M2 =
1000Kb 𝑊2
ΔTb 𝑊1