Osmosis
Osmosis means passage of solvent molecules from lower to higher
concentration of the solute through a semipermeable membrane,
which is permeable to the solvent and impermeable to the solute.]
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Osmotic pressure
It is the hydrostatic pressure needed to prevent osmosis i.e. it is the
pressure that should be exerted on the solution to prevent the passage
of solvent separated from it by semipermeable membrane.
Factors affecting osmotic pressure
1- Osmotic pressure is directly proportional to the concentration of
solute.
2- It is inversely proportional to the volume.
3- It is directly proportional to temperature.
4- It is directly proportional to the number of particles in the
solution.
Semipermeable
membrane
Solvent
Biological significance of osmotic
pressure
Colloidal
particles
1- Urine formation
Inside the glomerular capillaries, there are 2
Osmosis
opposing forces:
A- Filtration force caused by capillary blood pressure. It equals
+35 mm mercury (Hg)
B- Reabsorptive force caused by the osmotic pressure of plasma
proteins (-20 mmHg).
The net filtration pressure = +35 - 20 = +15 mm Hg.
2- Formation and reabsorption of interstitial fluid
Interstitial fluid is formed by filtration of blood plasma at the arterial
end of the blood capillaries and reabsorbed at the venous end of the
capillaries.
3- Haemolysis
RBCs are isotonic with 0.9% NaCl solution.
If the RBCs are put in hypertonic solution, they will loose water,
shrink and become crenated.
If the RBCs are put in hypotonic solution they will absorb water, swell
and rupture i.e. they become haemolyzed.
4- Maintenance of blood volume by the osmotic pressure of plasma
proteins
5- Magnesium sulphate is used as laxative as it absorbs water by its
osmotic pressure forming bulky stools.
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Measurement of osmotic pressure
We can measure osmotic pressure by:
1- Direct means by osmometer.
Column of Mercury
Solution
Distilled
water
Semipermeable
membrane
Osmometer
2- Indirect means by measuring changes in the boiling and
freezing points of a solvent after addition of the solution, which
we want to measure its osmotic pressure.
Expression Of Concentration
The concentration of a solution is defined as the amount of solute
dissolved in a given amount of solvent or solution.
Amount of solute
Concentration =
Amount of
There are many ways in which the concentration of a solution can be
described.
1- The molarity (M) of a solution is defined as the number of
moles of solute in the solution divided by the volume of the
solution in liters.
Molar solution is a solution, which contains the molecular weight of
the solute in grams dissolved in one liter of the solution.
Usually the concentration is expressed as a fraction of the mole e.g.
Millimole (mM) = 1/1000 mole
Micromole (υM) = 1/1000,000 mole
Nanomole (nM) = 1/1000,000,000 mole
Molarity is the concentration unit most commonly used by chemists. It
has one disadvantage. It tells us how much solute we need to make a
solution, and it gives us the volume of the solution produced, but it
doesn't tell us how much solvent will be required
When it is important to know how much solute and solvent are present
in a solution, we use two other concentration units; molality and mole
fraction.
2- The molality (m) of a solution is defined as the number of moles of
solute in the solution divided by the mass in kilograms of the solvent.
Notice that since the molality of a solution is determined by the
masses of solute and solvent, it is temperature independent.
Molality has an important advantage over molarity. The molarity of an
aqueous solution changes with temperature, because the density of
water is sensitive to temperature. Because molality is defined in terms
of the mass of the solvent, not its volume, the molality of a solution
does not change with temperature.
Molal solution is a solution, which contains the molecular weight of
the solute in grams dissolved in one kilogram of the solvent.
3- Normality (N): the normality of a solution is defined as the number
of equivalents of solute per liter of solution. The equivalent equals the
molecular weight in grams divided by the valence.
Normal solution is a solution, which contains the equivalent weight of
the solute in grams dissolved in one liter of the solution.
4- Mass percent is literally the percentage of the total mass of a
solution that is due to the solute.
A 3.5% solution of hydrochloric acid, for example, has 3.5 grams of
HCl in every 100 grams of solution..
5- Volume percent
It is also possible to describe the concentration of a solution in terms
of the volume percent. This unit is used to describe solutions of one
liquid dissolved in another or mixtures of gases.
6- Mole fraction (Χ)
The ratio of solute to solvent in a solution can also be described in
terms of the mole fraction of the solute or the solvent in a solution.
Mole fraction has no units.
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• The mole fraction of a solute (Χ Solute ) in a solution is equal
to the number of moles of solute divided by the total number of
moles in the solution (moles of solute plus moles of solvent.) If
the solution consists of more than one solute, then the
denominator is the sum of the moles of all components.
• The mole fraction of a solvent (Χ Solvent) is the number
of moles of solvent divided by the total number of moles
of solute and solvent.
• Mole percentage is simply mole fraction multiplied by 100.
7- Parts per million (ppm)
A unit of concentration often used when measuring levels of
pollutants in air, water, body fluids, etc. One ppm is 1 part in
1,000,000. The common unit mg/liter is equal to one ppm.
N.B. :
• The weight of a molecule is the sum of the weights of the atoms
of which it is made.
• The unit of weight is the dalton (da) which equals one-twelfth
(1/12) of the weight of an atom of 12C. Thus the molecular
weight (MW) of water is 18 daltons and that of glucose is 180
daltons.
• The mole is the quantity of a substance whose weight in grams
is equal to the molecular weight of the substance. Thus 1 mole
of glucose weighs 180 g. Furthermore, if you dissolve 1 mole of
a substance in enough water to make 1 liter (L) of solution, you
have made a 1 molar (1 M) solution.
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