Chemistry – Class XII – Solutions – Revision Notes – Types of Solutions, Units of Concentration, Vapour Pressure & Raoults Law, Ideal & Non-Ideal Solutions, Colligative Properties, Abnormal Molecular Mass & Vant Hoff Factor – Very Useful for Students stud

Solutions (REVISION NOTES)
BY ANKUR SIR
1
CHAPTER
Different methods for expressing concentration of solution - molality, molarity,
mole fraction, percentage (by volume and mass both), vapour pressure of
solutions and Raoult’s Law - Ideal and non-ideal solutions, vapour pressure -
composition, plots for ideal and non-ideal solutions; Colligative properties
of dilute solutions - relative lowering of vapour pressure, depression of freezing
point, elevation of boiling point and osmotic pressure; Determination of




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molecular mass using colligative properties; Abnormal value of molar mass,
van’t Hoff factor and its significance.

SOLUTIONS
A homogeneous mixture of two or more non-reacting substances is known as
solution. Homogeneity or heterogeneity depends upon particle size and states of
matter present in the solution. Every solution is made up of a solvent (present in
larger quantity) and one or more solute (present in smaller quantity).

TYPES OF SOLUTION
LA
A. Gaseous Solutions
(i) Gas in gas
(ii) Liquid in gas
(iii) Solid in gas
B. Liquid Solutions
(i) Gas in liquid
A
(ii) Liquid in liquid
(iii) Solid in liquid
C. Solid Solutions
(i) Gas in solid
(ii) Liquid in solid
B

(iii) Solid in solid

Note : In solution chapter we mostly deal with solid in liquid or liquid in
liquid.


UNITS OF CONCENTRATION
(i) Molarity (M)
It is the no. of moles of solute present per litre of solution.
n w W  1000
M  
V M  Vin litre M  Vin cc




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, W
M × Vin cc =  1000
M
mM = millimoles
Molarity changes with temperature of the solution. Increase in temperature decreases the molarity. It is the
most convenient method to express concentration of the solution. On dilution molarity decreases.
(ii) Molality (m) :
No. of moles (n) of solute present per kg of solvent

n w w
m    1000
W in kg M  Win kg M  Win g solvent 

It is independent of temperature since no volume factor is involved in the equation.




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(iii) Mole fraction (x)
It is the ratio of no. of moles of one component to the total no. of moles present in the solution.
For a system having two components A and B.

nA nB
XA  , XB 
n A  nB n A  nB


LA
X A  XB  1

Mole fraction is also independent of temperature.
(iv) In terms of %

wt. of solute
% by weight =  100
wt. of solution

wt. of solute
% by volume   100 (In case of solid dissolved in a liquid)
vol. of solution
A
volume of solute
% by volume   100 (In case of liquid dissolved in another liquid)
volume of solution

vol. of solute
% by strength   100
vol. of solution

% by weight is independent of temperature while % by vol., % by strength or strength are temperature
B

dependent.

VAPOUR PRESSURE AND RAOULT’S LAW
The pressure exerted by the vapours at the free surface of liquid provided system is closed is known as its
vapour pressure. The V.P. of a pure liquid is always greater than its solution (In case of non volatile solute).
(a) Raoult’s Law for a solution having non volatile solute

 x solute  mole fraction of solute in solution
P o  Ps  
X solute  P  V.P. of pure solvent 
Po Ps  V.P. of solution 
i.e relative lowering of vapour pressure is equal to the mole fraction of solute.




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