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BALAJI
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
A. Gaseous Solutions
(i) Gas in gas (ii) Liquid in gas
(iii) Solid in gas B. Liquid Solutions
(i) Gas in liquid (ii) Liquid in liquid
(iii) Solid in liquid C. Solid Solutions
(i) Gas in solid (ii) Liquid in solid
(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.
M V in cc
W 1000
M V
w V
n M in litre
Solutions (REVISION NOTES) C H A P T E R
BY ANKUR SIR
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 molecular mass using colligative properties; Abnormal value of molar mass, van’t Hoff factor and its significance.
BALAJI
M × V in cc = 1000 M
W
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
M W solvent
w M W
w W
n m
in kg inkg in g
It is independent of temperature since no volume factor is involved in the equation. (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.
A B
B B A B
A A n n
n ,X n n
n X
X A X B 1
Mole fraction is also independent of temperature. (iv) In terms of %
% by weight = 100 wt.ofsolution
wt. ofsolute
% by volume 100 vol.ofsolution
wt. ofsolute (In case of solid dissolved in a liquid)
% by volume 100 volumeofsolution
volume ofsolute (In case of liquid dissolved in another liquid)
% by strength 100 vol.ofsolution
vol.of solute
% by weight is independent of temperature while % by vol., % by strength or strength are temperature 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
o
s o
P
P P
X
solute
P V.P.of solution
P V.P.ofpuresolvent
x molefractionofsoluteinsolution
s
solute
i.e relative lowering of vapour pressure is equal to the mole fraction of solute.
BALAJI
(a) Relative lowering of V.P. : The relative lowering in V.P. of an ideal solution is equal to the mole fraction of solute at that temperature.
A
A A p
p p = B =^12
2 n n
n =^1
2 n
n
1
1 2
2 w
M
M
w
Determination of molecular masses by relative lowering in vapour pressure.
W
M
m
w p
p p A
A
w = wt. of solute m = Mol. wt. of solute W = wt. of solvent M = Mol. wt. of solvent
(b) Osmotic pressure : The excess pressure which must be applied on a solution to prevent the passage of solvent into it through a semipermeable membrane.
Determination : Barkley–Hartley method:
Semipermeable membrane egg membrane;
Chemical Semipermeable membrane cupric ferrocyanide.
= CRT = n / V. RT; V = nRT + van't Hoff equation for dilution solutions
2
2 M
w n (^) ; V
w.RT M
2 ^^2
(c) Elevation in boiling point: The property of rise in boiling point when some non volatile solute is added.
We know that the vapour pressure of the solution is lower
V
a po
u r^ pr
e ss
u re
Temperature
Atmospheric Pressure Solvent
Solution
A
B
C
D
E
F
T b
T O T
Elevation in boiling point
than that of the pure solvent and vapour pressure increases with increase in temperature. Hence the solution has to be heated more to make the vapour pressure equal to the atmospheric pressure.
Alternatively, the elevation in boiling point may be explained on the bais of plots of vapour pressure versus temperature as follows :
Vapour pressure of the solvent increases with increase in temperature as shown by the curve AB. As at any temperature, vapour pressure of the solution is less than that of the solvent, the curve for the solution lies below that of the solvent, as shown by the curve CD. The temperatures at which the vapour pressure of the solvent and the solution become equal to the atmospheric pressure are T 0 and T respectively. Obviously T > T 0. The difference, called the elevation in boiling point, T b , is given by
T b = T – T 0
Molal elevation constant or ebulioscopic constant, kb. It is the increase in boiling point when the molality of the solution is unity.
Tb = kbm when m = 1, Tb = kb
MB = b b A
B (^) k T W
W 1000
BALAJI
(d) Depression in freezing point : The property of decrease in freezing point when some non-volatile solute is dissolved. The depression in freezing point is given by T f Freezing point : Temperature at which the liquid and the solid forms of the same substance are in equilibrium and hence have same vapour pressure.
We know that vapour pressure of the solution is less than that of the pure solvent. As freezing point is the temperature at which the vapour pressure of the liquid and the solid phase are equal, therefore for the solution, this will occur at lower temperature (lower the temperature lower the vapour pressure). The graph explains this.
V
ap
ou
r^ p re
ss
u re
Temperature
T f T ° f
Solid solvent
Liquid solvent
Tf = T° f – T f Molal depression constant. or cryoscopic constant (kf). It is the decrease in freezing point when the molality of solution is unity
Tf = kf.m
when m = 1, Tf = kf
MB = f f A
B (^) 1000 k T W
W
Kb and Kf are intensive property of solvent and doesnot depend upon solute or solution.
ABNORMAL MOLECULAR MASS AND van’t HOFF FACTOR ( i )
Calculatedvalueofcolligative property
ExperimentalvaluesofColligativeproperty i
Normalvalueofthesame property
ObservedvalueofColligativeproperty
M O
M
Observedmoleculermass
Normal moleculer mass C
Since Colligative property molecularmassofsolute
if i = 1, no molecular association or dissociation takes place if i < 1, molecular association takes place
if i > 1, molecular dissociation takes place. For substances undergoing association or dissociation in the solution.
ΔT iKb m
ΔT iKf m
i CRT
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