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152 Solution and Colligative propertiesChapter
4
Solution & Colligative properties
“A solution is a mixture in which substances are Liquid Solid Salt in water, sugar in water.
intermixed so intimately that they can not be observed Solid Gas Adsorption of gases over metals;
as separate components”. The dispersed phase or the hydrogen over palladium.
substance which is to be dissolved is called solute, Solid Liquid Mercury in zinc, mercury in gold,
CuSO4.5H2O.
while the dispersion medium in which the solute is
Solid Solid Homogeneous mixture of two or more
dispersed to get a homogenous mixture is called the metals (alloys), e.g., copper in gold,
solvent. zinc in copper.
Solubility Among these solutions the most significant type
of solutions are those which are in liquid phase and
“Solubility of a substance may be defined as the
may be categorised as, (1) Solid in liquid solutions,
amount of solute dissolved in 100gms of a solvent to
(2) Liquid in liquid solutions and (3) Gas in liquid
form a saturated solution at a given temperature”. A solutions.
saturated solution is a solution which contains at a
Methods of expressing concentration of solution
given temperature as much solute as it can hold in
presence of dissolveding solvent. Any solution may Concentration of solution is the amount of solute
contain less solute than would be necessary to saturate dissolved in a known amount of the solvent or solution.
it. Such a solution is known as unsaturated solution. The concentration of solution can be expressed in
When the solution contains more solute than would be various ways as discussed below,
necessary to saturate it then it is termed as (1) Percentage : It refers to the amount of the
supersaturated solution. solute per 100 parts of the solution. It can also be
called as parts per hundred (pph). It can be expressed
Kinds of solutions by any of following four methods,
All the three states of matter (gas, liquid or solid) (i) Weight to weight percent
may behave either as solvent or solute. Depending on Wt. of solute
% w/w 100
the state of solute or solvent, mainly there may be Wt. of solution
following nine types of binary solutions. Example : 10% Na 2 CO 3 solution w/w means 10 g
Solvent Solute Example of Na 2 CO 3 is dissolved in 100 g of the solution. (It
Gas Gas Mixture of gases, air. means 10 g Na 2 CO 3 is dissolved in 90 g of H 2 O )
Gas Liquid Water vapours in air, mist.
Gas Solid Sublimation of a solid into a gas,
(ii) Weight to volume percent
smoke. Wt. of solute
% w/v 100
Liquid Gas CO2 gas dissolved in water (aerated Volume of solution
drinks). Example : 10% Na 2 CO 3 (w/v) means 10 g Na 2 CO 3
Liquid Liquid Mixture of miscible liquids, e.g.,
alcohol in water.
is dissolved in 100 cc of solution.
(iii) Volume to volume percent
, Solution and Colligative properties 153
Vol. of solute Wt. of solute per litre of solution
% v/v 100 (iii) N
Vol. of solution g eq. wt. of solute
Example : 10% ethanol (v/v) means 10 cc of
Wt. of solute 1000
ethanol dissolved in 100 cc of solution. (iv) N
g.eq. wt. of solute Vol. of solution in ml
(iv) Volume to weight percent
Percent of solute 10
Vol. of solute (v) N ,
% v/w 100 g eq. wt. of solute
Wt. of solution
Example : 10% ethanol (v/w) means 10 cc of Strength in g l -1 of solution
(vi) N
ethanol dissolved in 100 g of solution. g eq. wt. of solute
(2) Parts per million (ppm) and parts per Wt% density 10
(vii) N
billion (ppb) : When a solute is present in trace Eq. wt.
quantities, it is convenient to express the concentration
(viii) If volume V1 and normality N1 is so
in parts per million and parts per billion. It is the
number of parts of solute per million (10 6 ) or per changed that new normality and volume N 2 and V2
then,
billion (10 9 ) parts of the solution. It is independent of
N 1 V1 N 2 V2 (Normality equation)
the temperature.
mass of solute component (ix) When two solutions of the same solute are
ppm 10 6 mixed then normality of mixture (N ) is
Total mass of solution
N 1 V1 N 2 V2
mass of solute component N
ppb 10 9 V1 V2
Total mass of solution
(x) Vol. of water to be added i.e., (V2 V1 ) to get a
(3) Strength : The strength of solution is defined
solution of normality N 2 from V1 ml of normality N 1
as the amount of solute in grams present in one litre
(or dm 3 ) of the solution. It is expressed in g/litre or N N2
V2 V1 1 V1
N2
3
(g / dm ) .
M ass of solute in grams (xi) If Wg of an acid is completely neutralised by
Strength
Volume of solution in litres V ml of base of normality N
(4) Normality (N) : It is defined as the number of Wt. of acid VN
gram equivalents (equivalent weight in grams) of a g eq. wt. of acid 1000
solute present per litre of the solution. Unit of Wt. of base Vol. of acid N of acid
normality is gram equivalents litre–1. Normality Similarly,
g eq. wt. of base 1000
changes with temperature since it involves volume.
(xii) When Va ml of acid of normality N a is mixed
When a solution is diluted x times, its normality also
decreases by x times. Solutions in term of normality with Vb ml of base of normality N b
generally expressed as, (a) If Va N a Vb N b (Solution neutral)
N Normal solution; 5 N Penta normal, (b) If Va N a Vb N b (Solution is acidic)
10 N Deca normal; N / 2 semi normal (c) If Vb N b Va N a (Solution is basic)
N / 10 Deci normal; N / 5 Penti normal (xiii) Normality of the acidic mixture
N / 100 or 0.01 N centinormal, Va N a Vb N b
(Va Vb )
N / 1000 or 0.001= millinormal
(xiv) Normality of the basic mixture
Mathematically normality can be calculated by Vb N b Va N a
following formulas,
(Va Vb )
Number of g.eq. of solute
(i) Normality ( N ) (xv) N
No. of meq * of solute
Volume of solution (l) Vol. of solution in ml
Weight of solute in g. (* 1 equivalent = 1000 milliequivalent or
(ii) N
g. eq. weight of solute Volume of solution (l) meq.)
(5) Molarity (M) : Molarity of a solution is the
number of moles of the solute per litre of solution (or
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