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Chemistry Notes for class 12 Chapter 2
Solutions
Solution is a homogeneous mixture of two or more substances in same or different physical
phases
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On the basis
of number of components a solution of two components is called binary solution
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Classification of Solutions
(A) Following types of solutions are seen on the basis of physical state of solute and solvent
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]
(B) Depending upon the amount of solute dissolved in a solvent we have the following types of
solutions:
(i) Unsaturated solution A solution in which more solute can be dissolved without raising
temperature is called an unsaturated solution
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(iii) Supersaturated solution A solution which contains more solute than that would be
necessary to saturate it at a given temperature is called a supersaturated solution
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The solubility of a solute in a liquid depends upon the following factors:
(i) Nature of the solute
(ii) Nature of the solvent
(iii) Temperature of the solution
(iv) Pressure (in case of gases)
Henry’s Law
The most commonly used form of Henry‟s law states “the partial pressure (P) of the gas in
vapour phase is proportional to the mole fraction (x) of the gas in the solution” and is expressed
as
p = KH
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The value of KH decreases with
increase in the temperature
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Applications
1
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2
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O2diluted with less soluble
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3
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This leads to
low concentrations of O2 in the blood of climbers which causes „anoxia‟
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On the basis of concentration of solution there are two types of solutions
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w / w % = weight of solute / weight of solution * 100
(ii) Percentage by volume (w / V%) It is defined as the weight 01 solute present in 100 mL of
solution
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u / V % = volume of solute / volume of solution * 100
(iii) Mole fraction (x) It is defined as the ratio of the number of moles of a component to the
total number of moles of all the components
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It is widely used when a solute is present in trace quantities
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M = number of moles of solute / volume of solution (L)
M = mass of solute (in gram) * 1000 / mol
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of solute x volume of solution (in mL)
Molarity varies with temperature due to change in volume of solution
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0
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5 M solution is known as semi molar solution]
Molarity = Percent by mass * density * 10 / molecular weight
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Dilution law, M1 V1 = M2 V2 (for dilution from volume V1 to V2)
For reaction between two reactants, M1 V1 / n1 = M2 V2 / n2
where, n1 and n2 arc stoichiometric coefficient in balanced equation
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Molality = mass of solute in gram * 1000 / mol
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of solute * mass of solvent (in g)
Molality is independent of temperature
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]
(vii) Normality (N) The number of gram equivalents of solute present in 1 L of solution
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weight = M x mol
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Formality = moles of substance added to solution / volume of solution (in L))
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(ix) Mass fraction Mass fraction of any component in the solution is the mass of that
component divided by the total mass of the solution
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, because
former involve weights which do not change with temperature
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Raoult’s Law
The Raoult‟s law states “For a solution of two volatile liquids, the vapour pressure of each
liquid in the solution is less than the respective vapour pressure of the pure liquids and the
equilibrium partial vapour pressure of the liquid is directly proportional to its mole fraction
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In other words, mole fraction of the more volatile component
is always greater in the vapour phase than in the solution phase
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If Y1 and Y2 are the
component 1 and 2 respectively in the vapour phase then
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These solutions satisfy
the following conditions :
(i) Solution must obey Raoult‟s law, i
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,
(ii) ΔHmix = 0 (No energy evolved or absorbed)
(iii) ΔVmix = 0 (No expansion or contraction on mixing)
Some solutions behave like nearly ideal solutions, e
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, benzene + toluene
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Non-ideal Solutions
Those solutions which shows deviation from Raoult‟s law is called non-ideal solution
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For such solutions
(b) Non-ideal solution showing negative deviation In such a case, the A – B interactions are
stronger than A – A or B – B interactions and the observed vapour pressure of each component
and the total vapour pressure are lesser than that predicted by Raoult‟s law
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These are formed by nonideal solutions
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Such azeotropes have boiling points lower than either of the
components, e
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, C2H5OH (95
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43%)(by mass)
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Such azeotropes have boiling points higher than either of the
components
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g
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22O%)+ HCl (79
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Colligative Properties
[Colligatil1e : from Latin
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]
Colligative properties are those properties which depends only upon the number of solute
particles in a solution irrespective of their nature
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The relative
lowering in vapour pressure of solution containing a nonvolatile solute is equal to the mole
fraction of solute in the solution
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Where, WB and WA = mass of Solute and solvent respectively
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Ostwald and Walker method is used to determine the relative lowering of vapour pressure
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As the vapour pressure of a solution containing a nonvolatile solute is
lower than that of the pure solvent, it boiling point will be higher than that of the pure solvent
as shown in figure
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Molecular mass of solute can be
calculated as
where, WB and WA = mass of solute and solvent respectively
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52 K kg mol-1
The boiling point elevation of a solution is determined by
(i) Landsberger‟s method
(ii) Cottrell‟s method
Depression in Freezing Point (ΔTf)
Freezing point of a liquid is the temperature at which vapour pressure of the solvent in its
liquid and solid phase become equal
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This decrease in freezing point of a liquid is known as depression in freezing point
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Kf has units of K / m or K kg mol-1
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It is called
antifreeze solution
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The freezing point depression is determined by Beckmann
method or Rast method
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It was first observed by Abbe Nollet
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CU2[Fe(CN)6]is an artificial semipermeable membrane which does not work in non-aqueous
solutions as it dissolves in them
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(ii) Endosmosis It is inward flow of water or solvent from a cell through a semipermeable
membrane
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where, d = density, R = solution constant,
T = temperature, MB = molar mass of solute
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Osmotic pressure can be determined by anyone of the method listed below
(i) Pfeffer‟s method
(ii) Berkeley and Hartley‟s method (very good method)
(iii) Morse and Frazer‟s method
On the basis of osmotic pressure, -the solution can be
(i) Hypertonic solution A solution is called hypertonic if its osmotic pressure is higher than
that of the solution from which it is separated by a semipermeable membrane
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(ii) Hypotonic solution A solution is called hypotonic if its osmotic pressure is lower than that
of the solution from which it is separated by a semipermeable membrane
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These solutions have same molar concentration
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91% solution of pure NaCl is
isotonic with human RBC‟s
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g
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Reverse osmosis When the external pressure applied on the solution is more than osmotic
pressure, the solvent flows from the solution to the pure solvent, I which is called reverse
osmosis
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Abnormal Molecular Masses
In some cases, observed colligative properties deviate from their normal calculated values due
to association or dissociation of molecules
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g
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Similarly KCI undergoes dissociation in aqueous solution, so its
observed molecular mass is 37
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These observed values are corrected by multiplying with van‟t Hoff factor (i)
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i = observed value of colligative property / calculated value of colligative property
or i = normal molecular mass / observed molecular mass
or i = number of particles after association or dissociation / number of particles initially
So to correct the observed value of molar mass, van‟t Hoff factor (i) must be included in
different expressions for colligative properties
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