Solution - I

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Solution - I
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Session objectives

• Introduction
• Solubility
• Henrys law
• Different concentration terms
• Vapour pressure
• Raoults law and its modification
• Relative lowering of vapour pressure
• Ideal solutions and non-ideal solutions
• Maximum and minimum boiling solutions

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Introduction
Solute Component of solution present in smaller
amount.
Solvent Component of solution present in the
larger amount.
Solution a homogenous mixture of two or more
substances.

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Solubility
Maximum amount of solute in grams which can be
dissolved in a given amount of solvent (generally
100 g) to form a saturated solution at that
particular temperature is known as its solubility

• For solids
• Solubility of ionic compounds in water generally
  increases with increase in temperature.

• For gases
• The solubility of gases in water decreases with
  increase in temperature.
• Solubility tends to zero at the boiling point of
  water.

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Effect of pressure on solubility of gases

• Increase in pressure of the gas above the
  solutionincreases the solubility of the gas in
  the solution.

More dilute solution
More concentrated solution
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Henrys law
Solubility of a gas in a liquid is proportional
to the pressure of the gas over the solution.
C kP C molar concentration, P pressure,
k temperature-dependent constant
Carbonated cold drink is an application of
Henrys law.
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Different concentration terms
Molarity of a solution changes with temperature
dueto accompanied change in volume of the
solution.
x1mole fraction of solvent x2mole fraction of
solute
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Illustrative Example
Determine the molality of a solution prepared by
dissolving 75 g of Ba(NO3)2(s) in 374 g of water
at 25oC.
Solution
Molar mass of Ba(NO3)2 261
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Illustrative Example
Calculate the molality of 1 molar solution of
NaOH given density of solution is 1.04 gram/ml.
Solution
1 molar solution means 1 mole of solute present
perlitre of solution.
Therefore, mass of 1 litre solution 1000 x 1.04
1040 gram
Mass of solute 1 x 40 40g
Therefore, mass of solvent 1040 40 1000g
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Different concentration terms
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Illustrative Example
Calculate the concentration of 1 molal solution
of NaOH in terms of percentage by mass.
Solution
1 molal solution means 1 mole (or 40g) NaOH
present in 1000g of solvent.
Total mass of solution 1000 40 1040g
Therefore, 1040g solution contains 40g NaOH
3.84 by mass.
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Different Concentration terms
Relation between Molarity (M) and molality (m)
Relation between molality(m) and mol-fraction
(x2) of solute
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Illustrative Example
Calculate the molality and mole fraction of the
solute in aqueous solution containing 3.0 g of
urea (molecular mass 60) in 250 g of water.
Solution
Mole fraction of urea
Mole fraction of water 1 0.00359 0.996
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Illustrative Example
Calculate the mol fraction of ethanol and water
in a sample of rectified spirit which contains
95 of ethanol by mass.
Solution
95 of ethanol by mass means 95 g ethanol present
in 100 g of solution. Hence, mass of water 100
95 5 g
Mole fraction of water 1 0.88 0.12
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Vapour pressure of solution
vapour pressure of pure liquid
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Factors affecting Vapour Pressure
Nature of liquid More volatile liquids exert
more pressure on the liquid surface.
Temperature Increase in temperature
increases vapour pressure.
Presence of a solute Due to presence of
volatile and non-volatile solute, vapour
pressure of solution decreases.
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Vapor Pressure of Solution

• Some of the solute particles
• will be near the surface.

• Block solvent molecules
• from entering the gas phase.

• Less no. of molecules per
• unit surface area are
• involved in equilibrium.

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Raoults law for non-volatile solute
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Raoults law for non-volatile solute
Applicable for ideal solution
Here, solute-solute and solvent-solvent
interaction exactly equal in magnitude with
solute-solvent interaction.
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Relative lowering of vapour pressure
From Raoults law,
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Modification (two volatile liquids)
According to Raoults law,for two volatile
miscible liquids
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Modification (two volatile liquids)
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Illustrative Example
Vapour pressure of liquids A and B at a
particular temperature are 120 mm and 180 mm of
Hg. If 2 moles of A and 3 moles of B are mixed to
form an ideal solution, what would be the vapour
pressure of the solution?
Solution
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Illustrative Problem
At 40oC, the vapour pressure in torr of methyl
alcohol-ethyl alcohol solution is represented by
P 119Xm 135 where Xm is the mole fraction
of methyl alcohol. What are the vapour pressures
of pure methyl alcohol ethyl alcohol ?
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Solution
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Illustrative Problem
6g of urea is disolved in 90g water at 25oC ?
What is vapour pressure of sol. If vapour
pressure of water is 40mmHg.
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Solution
ps po x solvent
ps 0.980 x 40 39.2 mm Hg
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Modification (two volatile liquids)
From Daltons law of partial pressure
yAmol. fraction of A in vapour phase
psvapour pressure of solution.
From (2)
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Modification (two volatile liquids)
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Illustrative Problem
An unknown compound is immiscible with water. It
is steam distilled at 98.0oC and P 737
Torr.poH20 707 torr at 98.0oC. This
distillate was 75 by weight of water. Calculate
the molecular weight of the unknown
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Solution
Using Daltons law of partial pressure Ptotal
737 torr PoH2O 707 torr Pounknown 737
707 30 torr.
If water 100 g the unknown 75.0 g
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Non-ideal solution
Solute-solvent interaction are different than
solute-solute and solvent solvent in non ideal
solutions.

These do not obey Raoults Law.
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Non ideal solution

• For solution showing negative deviation from
  Raoult's law.

• For solution showing positive
• deviation from Raoult's law.

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Azeotropic mixtures
Liquid mixtures which distil without any change
in composition are called Azeotropes or
Azeotropic mixtures.
Solution showing positive deviation from Raoults
form minimum boiling azeotrope
Interaction between AB lt interaction between AA
or BB
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Azeotropic mixtures
Solution showing negative deviation from
Raoults law form maximum boiling azeotropes
Interaction between A B gt interaction between A
A or B B
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