Chemistry final notes NCERT and HSC combine
Solutions
Solutions are homogeneous mixtures of two or more than two
components. By homogenous mixture we mean that its
composition and properties are uniform throughout the mixture.
The component that is present in the largest quantity is known as
solvent.
One or more components present in the solution other than
solvent are called solutes.
(i) Mass percentage (w/w): The mass percentage of a component
of a solution is defined as:
Mass % of a component =
Mass of the component in the solution
______________________________ X 100
Total mass of the solution
(ii) Volume percentage (V/V): The volume percentage is defined
as:
Volume % of a component =
Volume of the component
____________________________ X 100
Total volume of solution
(iii) Parts per million =
Number of parts of the component
________________________________x106
Total number of parts of all
components of the solution
(iv) Mole fraction of a component =
Number of moles of the component
____________________________________
Total number of moles of all the components
xA = NA/[ NA + NB ]
Molality (m) = Moles of solute
_______________________
Mass of solvent in kg
1.3 Solubility
Solubility of a substance is its maximum amount that can be
dissolved in a specified amount of solvent at a specified
temperature. It depends upon the nature of solute and solvent as
well as temperature and pressure.
Solubility of a Solid in a Liquid
It is observed that polar solutes dissolve in polar solvents and
non polar solutes in nonpolar solvents. In general, a solute
dissolves in a solvent if the intermolecular interactions are
similar in the two or we may say like dissolves like.
Some solute particles in solution collide with the solid solute
particles and get separated out of solution. This process is
known as
crystallisation.
Solute + Solvent ⇌ Solution
Such a solution in which no more solute can be dissolved at the
same temperature and pressure is called a saturated solution. An
unsaturated solution is one in which more solute can be dissolved
at the same temperature.
If in a nearly saturated solution, the dissolution process is
endothermic (Dsol H > 0), the solubility should increase with
rise in temperature and if it is exothermic (Dsol H < 0) the
solubility should decrease.
Solubility of gases in liquids is greatly affected by pressure and
temperature. The solubility of gases increase with increase of
pressure.
Henry was the first to give a quantitative relation between
pressure and solubility of a gas in a solvent which is known as
Henry’s law. The law states that at a constant temperature, the
solubility of a gas in a liquid is directly proportional to the
partial pressure of the gas present above the surface of liquid or
solution.
Mole fraction of gas in the solution is proportional to the partial
pressure of the gas over the solution.
“The partial pressure of the gas in vapour phase (p) is
proportional to the mole fraction of the gas (x) in the solution”.
p = KH x
The solubility of gases increases with decrease of temperature.
Vapour Pressure of Liquid-Liquid Solutions
The relationship is known as the Raoult’s law which states that
for a solution of volatile liquids,the partial vapour pressure of
each component of the solution is directly proportional to its
mole fraction present in solution.
p1 µ x1
and p1 = p01 x1
p2 = p20x2
ptotal = p1 + p2
ptotal = x1p10 + x2p20
= (1 – x2) p10+ x2p20
= p10+ (p20– p10) x2
p1= y1 ptotal
p2= y2 ptotal
the pressure exerted by the vapours of the liquid over the liquid
phase is called vapour pressure.The decrease in the vapour
pressure of solvent depends on the quantity of non-volatile
solute present in the solution, irrespective of its nature.
Raoult’s law in its general form can be stated as, for any solution
the partial vapour pressure of each volatile component in the
solution is directly proportional to its mole fraction.
Ideal Solutions
obey Raoult’s law
/_\mix H = 0, /_\mixV = 0
no heat is absorbed or evolved
. In pure components, the intermolecular attractive interactions
will be of types A-A and B-B, whereas in the binary solutions in
addition to these two interactions, A-B type of interactions will
also be present.
Non-ideal Solutions
does not obey Raoult’s law
The vapour pressure is either higher or lower
If it is higher, the solution exhibits positive deviation and if it is
lower, it exhibits negative deviation
chloroform molecule is able to form hydrogen bond with acetone
molecule as shown.
Some liquids on mixing, form azeotropes which are binary
mixtures
having the same composition in liquid and vapour phase and
boil at
a constant temperature.
There are two types of azeotropes called minimum boiling
azeotrope and maximum boiling azeotrope
1.6 Colligative Properties and Determination of Molar Mass
These are: (1) relative lowering of vapour pressure of the solvent
(2) depression of freezing point of the solvent
(3) elevation of boiling point of the solvent and
(4) osmotic pressure of the solution. All these properties depend
on the number of solute particles irrespective of their nature
relative to the total number of particles present in the solution.
Such properties are called colligative properties
Relative Lowering of Vapour Pressure
The End
