Colligative properties of diluted
solutions
Colligative properties
Colligative properties of solutions depend on number of kinetic units of
system (molecules or ions) and don’t depend on identity of solute.
•Lowering of vapor pressure of solvent above
solution
•Elevation of boiling-point
•Diminishing of freezing-point of solutions
•Osmosis
Salt in water makes it boil hotter
and cook the pasta faster. Why?
Salt helps against icy roads by lowering the freezing
Osmosis in the kidney. How it works? point of the water in the snow and ice. Why? 1
Raoult's law
Raoult's law states that the vapor pressure of a solution is equal to the sum of the
vapor pressures of each volatile component if it were pure multiplied by the mole
fraction of that component in the solution.
P = PA0 x(A) + PB0 x(B)
Raoult’s law may be used only for so-called ideal solutions or highly
diluted real solutions whose properties approximate to ideal solutions.
Equipment for
distillation
2
Raoult's law
if the dissolved substance (B) is not volatile
P = PA0 x(A) + PB0 x(B)
P = PA0 − P = PA0 (1 − x(A)) = PA0 x(B)
P
0
= x(B) François-Marie Raoult 1886
PA
Partial lowering of vapor pressure of solvent above solution is equal to
mole fraction of the dissolved substance.
Lowering of vapor pressure of Elevation of boiling-point
solvent in nonvolatile substance
solution Diminishing of freezing-point
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Normal boiling-point and freezing-point
Phase diagram of pure water 4
Elevation of boiling-point and
diminishing of freezing-point
ΔTb
ΔTC
Phase diagram of water solution with a non-volatile solute 5
Elevation of boiling-point
According to Raoult’s law, boiling-point elevation of a solution is
proportional to the molality of the dissolved substance and also does not
depend on its nature.
Tb = Kb b( X )
where Kb is molal boiling-point elevation
constant (ebullioscopic constant) of
solvent, b(X) is molality of the solute.
Ebulliometry - it is a method of measurement of boiling-point elevation of
solutions (ΔTb). This method can be used for calculation of molar mass of
solute, degree of dissociation of weak electrolytes 6
Freezing-point depression
Freezing-point depression of a solution is proportional to the molality of the
dissolved substance and also does not depend on its nature
Tf = Kf b( X )
where Kcr is molal freezing-point
depression constant (cryoscopic constant)
of solvent, b(X) is molality of the solute.
Cryometry - it is a method of measurement of freezing-point depression of a
solution (ΔTf) This method can be used for calculation of molar mass of solute,
degree of dissociation of weak electrolytes. 7
Osmosis
One-sided diffusion of solvent molecules through a semipermeable
membrane aside more concentrated solution is called osmosis
= c R T
where c is molarity
of solute X,
R is universal gas
constant and
T is temperature (K)
Osmotic pressure is the external pressure upon a
solution, at which osmotic equilibrium (through
semipermeable membrane) between solution and pure
solvent is established.
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Colligative properties of diluted solutions of
electrolytes. Isotonic factor.
For solutions of electrolytes (salts, acids, bases) experimentally received values
ΔTf, ΔTb and π are much bigger than expected ones (calculated form formulas).
Vant Hoff has suggested applying the correction multiplayer named isotonic factor
i for the account of such derivations
Tf = i Kf b( X )
Tb = i Kb b( X )
= i c R T
Isotonic factor - the ratio of actual number of particles in solution after dissociation
and number of molecules (structural units) of initially dissolved substance.
⎯⎯
→
⎯ xA + yB
AxBy ⎯ z+ z−
n= x+ y
i = n + (1 − ) = 1 + (n − 1)
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Isotonic, hypertonic, hypotonic.
Solutions with osmotic pressure identical to standard solution are called isotonic,
with higher osmotic pressure - hypertonic, with lesser - hypotonic.
In medicine for the standard osmotic pressure of the human blood plasma at 37°C
is taken, which is equal to 7,7∙105 Pa (7,7 atm). The physiological solution - 0,9%
or 0,154 molar NaCl solution in water is isotonic to the human blood plasma.
= i c R T = 1.95 0.154 103 8.314 310 = 7.73 105
The total concentration of dissolved inorganic and organic substances in the form
of ions, molecules and colloidal particles which can create certain osmotic
pressure is called osmotic concentration (osmolarity or osmolality).
In physiology a unit “osmol”, is used. One osmol contains 6,02∙1023 particles.
Osmolarity is analogous to molarity and osmolality corresponds to molality.
Osmolality of the body liquids is equal 292± 12 mosm/kg.
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Isotonic, hypertonic, hypotonic.
isotonic solution no change hypotonic solution hemolysis
hypertonic solution plasmolysis
Isotonic Hypotonic Hypertonic
(a) Cells in dilute salt solution (b) Cells in distilled water (c) Cells in concentrated salt solution 11
Role of osmosis and osmotic pressure in
biological systems
Osmosis promotes the normal course of various physical and chemical processes:
hydrations and dissociations of substances, reactions of hydrolysis, reduction-
oxidation, etc. The osmotic pressure arising in organs and tissues causes a state of
tension of the cellular cover (turgor of cell).
Preservation of constant
osmotic pressure (isotonicity)
is an important property of the
organism. The ability of some
tissues (liver tissue, hypodermic
tissue) to accumulate/deposit
the surplus amounts of
water/salts, and also the
capacity of the organism to
remove quickly these
substances with urine and
sweat. Especially important role
in support of isotonicity belongs
to kidneys.
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Osmosis vs Reverse osmosis
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