Unit # 9..........Solutions and Concentrations
Definition
Homogenous Mixture and its Types
Mixtures having uniform composition
throughout and their component particles cannot be seen with naked eyes are
called Homogenous Mixture. Homogenous mixtures can be classified according to
the size of their constituent particles as:
1.
|
Solutions (TrueSolutions);
|
the
dispersed particles are of molecular size (0.1 – 1 nm).
|
2.
|
Colloids
(False Solutions);
|
the dispersed
particles are larger than molecules(2-1000 nm) but not large enough to settle
out
|
3.
|
Colloidal
suspensions;
|
the dispersed
particles are much larger than molecules(>1000nm)& settle down on
standing
|
Summary of
Solutions, Colloidal Solutions and Colloidal Suspensions
Kind of Mixture
|
Particle Size
|
Characteristics
|
Solutions
e.g.
Air, wine, sea water
|
0.1-1.0
nm
(0.2-2.0
nm)
|
Transparent
to light(lack of exhibiting tyndall effect),
Dialyzable
Non-filterable
Does
not separate on standing
|
Colloidal solutions
e.g.
Milk, whipped cream, butter
|
2.0-1000
nm
|
Murky
or opaque to light (exhibit tyndall effect)
Non-dialyzable
Non-filterable
Does
not separate on standing
|
Colloidal Suspensions
|
>
1000 nm
|
Murky
or opaque to light (exhibit tyndall effect)
Non-dialyzable
Filterable
Separates
on standing
|
Solution/True Solution and its Properties
Solution or True Solutions
A solution is a homogenous mixture or
single-phase mixture of two or more different substances (i.e. solute and
solvent) having uniform chemical composition and similar physical properties
throughout. A solution is a homogenous mixture or single phase mixture of the
molecules, atoms or ions of two or more different components. In other words it is a one phase mixture of
solute (smaller component) and solvent (larger component) having uniform composition
throughout. The size of constituent particles in solution is of molecular
dimension ranging between 0.1 to 1 nm. e.g. aqueous table salt solution,
aqueous sugar solution.
Solute
It is that component of solution which is
present comparatively in small quantity e.g. in 5% aqueous sugar solution,
sugar is solute as it is in smaller amount. The solute may be a solid, a liquid
or a gas.
Solvent
It is that component of solution which is
always present in greater amount in which solute is dissolved e.g. in 10%
aqueous table salt solution, water is solvent as it is in larger amount. The
solvent may be a solid, a liquid or a gas. [When a solution is made from two
substances of different phases, the solvent is the substance that is of the
same phase as a resulting solution. If a solution is formed from two substances
of the same phase, the solvent is conventionally the substance present in the
greater amount.]
Aqueous
Solution
The solution containing water as a solvent is
called aqueous [Aqua means water] solution denoted by (aq). Water is considered
to be universal solvent.
Non-Aqueous
Solution
The solutions that do not contain water as
solvent is called Non-Aqueous Solution.
Binary
Solutions
It is a solution comprising of only two
substances i.e. solute and solvent.
Properties
of Solution
1.In solutions, the size of constituent
particles (called Crystalloids) range to 0.1–1 nm (or 0.2– 2 nm).
2. The constituent particles of solution cannot
be seen with low power microscope.
3. The component particles of solution can pass
through a filter paper and cannot be separated by filtration (non-filterable).
4. The constituent particles do not settle down.
5. It is homogenous.
6. It is transparent.
7. It does not exhibit Tyndall effect i.e.
particles cannot scatter light.[Tyndall effect is the scattering of light by
colloidal particles. The explanation of this is that the colloidal particles
unlike the crystalloids of true solutions are large enough to block the path of
light rays and so scatter them. Thus crystalloids do not exhibit tyndall effect].
8. The constituent particles of solution can
diffuse through a medium. (Dialysis is the passage of both water and
crystalloid solutes through a semi-permeable membrane. Only solutes of true
solutions can be dialysed. As a result of this property, dialysis is
extensively used to separate mixtures of true and false solutions).
Types of
Solution According to State
There are 9 different types of solutions
according to the three states of solute and solvent.
S. No.
|
State of
|
State of
|
State of
|
Examples
|
1.
|
Gas
|
Gas
|
Gas
|
Air
(mixture of 78% N2, 21% O2 and 1% other gases), Water
gas (mixture of CO + H2 gas), coal gas.
|
2.
|
Liquid
|
Gas
|
Liquid
|
Carbonated/aerated
drinks (CO2 in water) like Pepsi etc., Air dissolved in water, ammonia gas in
water.
|
3.
|
Solid
|
Gas
|
Solid
|
H2 gas adsorbed over palladium
metal (called interstitial hydrides).
|
4.
|
Gas
|
Liquid
|
Gas
|
Rose
scent in air, cloud (water vapours in air), Steam.
|
5.
|
Liquid
|
Liquid
|
Liquid
|
Gasoline
(mixture of hydrocarbons), alcohol in water, vinegar
|
6.
|
Solid
|
Liquid
|
Solid
|
Sodium
amalgam (Mercury in sodium), dental amalgam (hg in Ag), water in jelly
powder.
|
7.
|
Solid
|
Solid
|
Gas
|
Smoke
(carbon particles in air).
|
8.
|
Liquid
|
Solid
|
Liquid
|
Sea
water (NaCl and other salts in water), Sugar in water.
|
9.
|
Solid
|
Solid
|
Solid
|
Metal
alloys e.g. 14 karat gold (Au and Ag). Brass (Cu and Zn), bronze (Cu and Sn),
solder (Sn and Pb), steel (C and iron), glass.
|
Colloidal Solutions or False Solutions and its
Properties
The solutions in which
the individual solute particles are larger than the particles of true solutions
with size range to 2.0 –1000 nm (but
not large enough to be seen by the naked eyes as in the case of the particles
in a suspension) are called False or
Colloidal Solutions. These solute particles are called colloids e.g. starch, albumen. In
colloids the particles are the dispersed
phase, which are spread throughout the dispersion medium. In colloidal solution, particles are too big to
dissolve but not large enough to settle out. In fact, colloidal solutions
are intermediate cases between suspension and true solutions.e.g. Milk, Butter,
Whipped Cream
Properties
of Colloidal Solutions
1. They are cloudy
or milky in appearance but look transparent on dilution.
2. They are non-filterable
and can pass through pores of filter paper.
3. Their particles do not separate on standing.
4. Their particles cannot diffuse through a medium and thus non-dialysable.
5. They exhibit Tyndall effect
(like suspensions), which is the scattering of light in all directions by colloidal particles. The explanation for
this is that the colloidal particles are large enough to block the path of light rays and so scatter them.
Colloidal Suspensions
Colloidal
Suspensions or Colloidal Dispersions are border-line (intermediate) cases
between suspensions and colloidal solutions. In colloidal suspension, the size
of the particles is greater than 1000 nm but particles are too small to be seen
and they appear to have dissolved in the medium. e.g. blood, paint, aerosol
sprays, fog, smoke, clouds.
Suspensions
Definition
Suspension is defined as a heterogeneous
mixture consists of visible particles, each of which contains many thousands or
even millions of molecules, surrounded by molecules of liquid. In suspension,
the size of dispersed particles is larger than 1000 nm. e.g. Mud or slime (a
suspension of fine particles of solid in small quantity of liquid).
Explanation
If fine sand is stirred in water, the
crystals do not dissolve, but even after several days some of the smallest
particles remain suspended, such a mixture is called a Suspension.
Properties
of Suspensions
1.
In suspensions, the size of
constituent particles is larger than
1000 nm.
2.
Their particles can be seen by low power microscope.
3.
Their components can be separated by filtration.
4.
Their particles settle down after some time.
5.
It is heterogeneous.
6.
It is not transparent.
7.
It does exhibits Tyndall effect i.e. particles can scatter light.
Concentration of Solution and its Units
The amount of solute present in a given
amount of solvent or solution is called Concentration of a Solution. The greater the amount of solute present, the
more will be the concentration of solution.
Concentration Units
The
concentration of solution may be expressed in two units:
(a) Physical units. (b) Chemical units.
(a) Physical
Units
The gram and volume relationship is called
Physical Unit. Percentage concentration is the example of physical units. e.g:
1
|
10%
|
2
|
10% W/V NaCl aqueous
solution means 10 g of NaCl is dissolved in enough water to get 100 ml (cm3)
of NaCl solution.
|
3
|
10% V/V NH3
solution means that 10 cm3 of NH3 is mixed with 90 cm3
of solvent to get 100 cm3 of NH3 solution.
|
(b) Chemical
Units
Following
are the chemical units of concentration:
1. Normality.
2. Molarity.
3. Molality.
4. Mole
Fraction.
Normality
Definition
The number of gram equivalent of a solute
dissolved per dm3 of solution is called Normality denoted by N.
Examples
1.
|
A normal solution is one
that contains one gram equivalent of substance in one liter (dm3)
of solution denoted by 1N.
|
|||||||||||||||||||||
2.
|
A seminormal
Solution is one which contains half fraction gram equivalent of substance
in one liter (dm3) of solution denoted by 0.5 or N/2.
|
|||||||||||||||||||||
3.
|
A decinoraml solution
is one which contains one tenth of gram equivalent of a substance in one
liter (dm3) of solution denoted by 0.1 or N/10.For example decinoraml
(0.1 N) solution of NaOH contains 4 g/litre.
|
|||||||||||||||||||||
4.
|
A centinormal solution is one which
contains one hundredth fraction of gram equivalent of a substance in one
liter (dm3 ) of solution denoted by 0.01 or N/100.For example
Centinormal (0.01 N) solution of NaOH contains 0.04 g/litre.
|
Formula
Molarity (Molar Concentration)
Definition
The number of moles of a solute dissolved per
dm3 (litre) of solution is called Molarity denoted by M. It is temperature dependent function.
1). A molar
solution is one that contains one mole of substance in one liter(dm3)
of solution denoted by 1M.for
example molar (1M) solution of NaOH contains 40 g/litre.
2). A Semimolar Solution is one which
contains half fraction mole of a
substance in one liter (dm3 ) of solution denoted by 0.5 or M/2.For
example Semimolar (0.5 M) solution of NaOH contains 20 g/litre.
3). A decimolar solution is one which
contains one tenth of mole of a substance in one liter (dm3) of solution
denoted by 0.1 or M/10.For example decimolar (0.1 M) solution of NaOH contains
4 g/litre.
4). A centimolar solution is one
which contains one hundredth fraction of mole of a substance in one liter (dm3)
of solution denoted by 0.01 or M/100.For example centimolar (0.01 M) solution
of NaOH contains 0.04 g/litre.
Examples
1.
|
Molar
|
(1 M)
|
solution of H2SO4
contains
|
98
|
g/litre
|
2.
|
Semimolar
|
(0.5 M)
|
solution of H2SO4
contains
|
49
|
g/litre
|
3.
|
Decimolar
|
(0.1 M)
|
solution of H2SO4
contains
|
9.8
|
g/litre
|
Formula
Molality (Molal Concentration)
Definition
The
number of moles of a solute dissolved per kilogram (1000 g) of a solvent is
called Molality denoted by m.
1). A molal solution is one that contains one mole of substance
in one liter (dm3) of solution denoted by 1m.
2). A Semimolal Solution
is one which contains half fraction mole of
a substance in one liter (dm3 ) of solution denoted by 0.5 or
m/2.
3.) A decimolal solution
is one which contains one tenth of mole of a substance in one liter (dm3
) of solution denoted by 0.1 or m/10.
4). A centimolal solution is one
which contains one hundredth fraction of mole of a substance in one liter (dm3
) of solution denoted by 0.01 or m/100.
Example
1.
|
Molal
|
(1 m)
|
Na2CO3 solution
contains
|
106
|
g/kg
of water
|
2.
|
molal
|
(1m)
|
NaOH solution of contains
|
40.
|
g/kg of water
|
3.
|
Semimolal
|
(0.5 m)
|
Na2CO3 solution
contains
|
53
|
g/kg of water
|
4
|
Semimolal
|
(0.5m)
|
solution of
NaOH contains
|
20
|
g/ kg of water
|
5
|
centimolal
|
(0.01 m)
|
solution of NaOH contains
|
0.04
|
g/ kg of water
|
6
|
decimolal
|
(0.1 m)
|
solution of
NaOH contains
|
4
|
g/ kg of water
|
Formula
Mole Fraction
Definition
The ratio of the number of moles of one
component divided by the total number of moles present in solution is called
Mole Fraction denoted by X.
Mole fraction =
|
For
example
In a solution of two components A and B, the
mole fractions Xa and Xb are expressed as:
Xa =
|
; And
|
Xb =
|
Where; na and nb are
the number of moles of A and B respectively. It is obvious that the sum of the
mole fractions of the components in a solution is always unity. i.e.Xa + Xb = 1
Equivalent weight
1. Equivalent weight of a substance is the
number of parts by weight or amount of a substance which will combine with or
displaces 1part by weight of hydrogen, or 8 parts by weight of
oxygen or 35.5 parts by weight of chlorine. It is expressed in
amu.
For
example
2Na
|
+
|
H2
|
¾¾¾¾®
|
NaH
|
||
2(23)
|
1(2)
|
|||||
46
equivalent
|
2 equivalent
|
|||||
2 equivalent of H2 = 46 amu of
Na
|
||||||
1 equivalent of H2 = 46/2 = 23
amu
|
||||||
Thus
equivalent weight of Na is 23 amu.
2. Equivalent weight of a substance
expressed in gram is called Gram equivalent weight or One gram equivalent. For
example;
1.
|
One
gram equivalent of Na
|
=
|
23
g
|
2.
|
One
gram equivalent of Ca
|
=
|
20
g
|
3.
|
One
gram equivalent of Al
|
=
|
9 g
|
4.
|
One
gram equivalent of HCl
|
=
|
36.5g
|
5.
|
One
gram equivalent of H2SO4
|
=
|
49g
|
3. Equivalent
weight of different substances is given by:
1.
|
Equivalent
weight of element
|
=
|
Atomic
weight / valency
|
2.
|
Equivalent
weight of acid
|
=
|
Molecular
weight/ basicity
|
3.
|
Equivalent
weight of base
|
=
|
Molecular
weight/ acidity
|
4.
|
Equivalent
weight of salt
|
=
|
Molecular
weight/ no of positive charge
|
5.
|
Equivalent
weight of oxidant
|
=
|
|
6.
|
Equivalent
weight of reductant
|
=
|
Molecular
weight/ no. of electrons lost
|
Equivalent weight of Acids
1. Equivalent weight of an acid is the
number showing how many parts by weight of an acid contains one
part
by weight of replaceable hydrogen i.e.
Equivalent
weight of acid
|
=
|
Molecular weight of the acid / Basicity
|
For
example
1
|
Equivalent
weight of HCl
|
=
|
36.5/1
|
=
|
36.5
|
2
|
Equivalent
weight of HNO3
|
=
|
63/1
|
=
|
63
|
3
|
Equivalent
weight of CH3COOH
|
=
|
60/1
|
=
|
60
|
4
|
Equivalent
weight of H2SO4
|
=
|
98/2
|
=
|
49
|
5
|
Equivalent
weight of H2C2O4.2H2O
|
=
|
126/2
|
=
|
63
|
2. Equivalent weight of an acid is the
number showing how many parts by weight of an acid neutralizes one gram
equivalent an alkali (base) e.g. equivalent weight of H2SO4
can be calculated as follows:
2KOH
|
+
|
H2SO4
|
¾¾¾¾¾¾¾¾¾¾¾¾®
|
K2SO4
|
+
2H2O
|
|
2(56)
|
1(98)
|
|||||
112g
|
2 gram equivalent
|
|||||
2 gram equivalent of KOH are neutralized by
98 g of H2SO4
|
||||||
1 gram equivalent of KOH is neutralized by 98/2 g of H2SO4 =
49g
|
||||||
Thus equivalent weight of H2SO4
is 49g.
Equivalent weight of Base
1. Equivalent weight of a base is the
number showing how many parts by weight of a base contains one part by weight
of replaceable OH-
For
example
1
|
Equivalent
weight of NaOH
|
=
|
40/1
|
=
|
40
|
2
|
Equivalent
weight of KOH
|
=
|
56/1
|
=
|
56
|
3
|
Equivalent
weight of NH4OH
|
=
|
35/1
|
=
|
35
|
4
|
Equivalent
weight of Ca(OH)2
|
=
|
74/2
|
=
|
37
|
2. Equivalent weight of a base is the
number showing how many parts by weight of an alkali neutralizes one gram
equivalent an acid. Equivalent weight of NaOH can be calculated as follows:
2NaOH
|
+
|
H2SO4
|
¾¾¾¾¾¾¾¾¾¾¾¾®
|
Na2SO4
|
+
2H2O
|
|
2(40)
|
1(98)
|
|||||
80g
|
2 gram equivalent
|
|||||
2 gram equivalent of H2SO4
are neutralized by 80 g of NaOH
|
||||||
1 gram equivalent of H2SO4 is
neutralized by 80/2 g of NaOH
= 40g
|
||||||
Thus equivalent weight of NaOH is 40g.
Equivalent weight of Compounds (salts)
1. Equivalent weight of a compound (salt)
is the number showing how many parts by weight of a compound (salt) contains
one, two or three positive charge on its cation.i.e
For
example
1
|
Equivalent
weight of Na2CO3
|
=
|
106/2
|
=
|
53
|
2
|
Equivalent
weight of K2CO3
|
=
|
138/2
|
=
|
69
|
3
|
Equivalent
weight of CaCO3
|
=
|
100/2
|
=
|
50
|
4
|
Equivalent
weight of NaHCO3
|
=
|
84/1
|
=
|
84
|
2. Equivalent weight of a compound (salt)
is the number showing how many parts by weight of a compound (salt) interacts
with the known chemical equivalent an element or a compound (whose equivalent
weight is known). Equivalent weight of Na2CO3 can be
calculated as follows:
Na2CO3
|
+
|
H2SO4
|
------>
|
Na2SO4
|
H2O + CO2
|
|
1(106)
|
1(98)
|
|||||
106g
|
2 gram equivalent
|
|||||
2 gram equivalent of H2SO4
reacts with 106g of Na2CO3
|
||||||
1 gram equivalent of H2SO4 reacts
with 106/2g of Na2CO3 = 53g
|
||||||
Thus equivalent weight of Na2CO3
is 53g.
Equivalent weight
of Oxidizing agent
The equivalent weight
of an oxidizing agent or oxidant is that weight of it which gains one electron.
It is obtained by dividing molecular weight by no of electrons gained per mole.
For
example
1. Equivalent
of an oxidant KMnO4 in acidic medium can be calculated as:
Equivalent weight of KMnO4 =
158/5 = 31.6
2. Equivalent
of an oxidant KMnO4 in basic medium can be calculated as:
Equivalent weight of KMnO4 =
158/3 = 52.66
Equivalent weight of reducing agent
The equivalent weight
of a reducing agent or reductant is that weight of it which donates one
electron. It is obtained by dividing molecular weight by no of electrons lost
per mole.
For
example
1). equivalent of a reducing agent FeSO4.7H2O
can be calculated as:
2). equivalent of a reducing agent Mohr’s
salt (FeSO4. (NH4)2SO4.6H2O
can be calculated as:
Equivalent weight of FeSO4.7H2O
= 392/1
= 392
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