Laws of Osmotic Pressure
Laws of Osmotic Pressure
From a study of the experimental results obtained by Pfeffer, van't Hoff showed that for dilute solutions-
A. The osmotic pressure of a solution at a given temperature is directly proportional to its concentration.
B. The osmotic pressure of a solution of a given concentration is directly proportional to the absolute temperature.
From the above findings, van't Hoff IN 1877 established the laws of osmotic pressure and pointed out that these were closely related to the gas laws.
Boyle-van't Hoff Law for Solutions
If π is the osmotic pressure and C its concentration, from (a) we can write π ∝ C, if temperature is
constant.
If the concentration of the solute is expressed in moles per litre and V is the volume of the solution that contains 1 mol of solute-
C = 1/V
Thus, π ∝ 1/V at constant temperature
This relationship is similar to the Boyle's law for gases and is known as the Boyle-van't Hoff law for solutions.
Charles-van't Hoff Law for Solutions
If T is the absolute temperature, from the statement (B), we can write-
π ∝ T, if temperature is constant
This relationship is similar to the Charles' Law for gases and is known as Charles-van't Hoff law for solutions.
Van't Hoff Equation for Solutions
As shown above the osmotic pressure (π) of a dilute solution is inversely proportional to the volume (V) containing 1 mole of the solute and is directly proportional to the absolute temperature (T).
This is,
π ∝ 1/V
π ∝ T
Combining the above two equations, van't Hoff gave the general relationship-
π V = R' T
where R' is a constant. He showed that this equation was parallel to the general Gas Equation (PV = RT), as the value of R' calculated from the experimental values of π, V, and T came out to be almost the same as of the Gas constant (R).
If n moles of solute are dissolved in V litres of solution, the above equation become-
π V = n R T
Avogadro-van't Hoff Law for Solutions
Writing Van't Hoff equation for two different dilute solutions-
π1V1 = n1RT1
π2V2 = n2RT2
where n1 and n2 are the number of moles (molecules) of the solute in V1 and V2 litres of the two
solutions.
If π1 = π2 and T1 = T2
Then from above two equations-
n1/V1 = n2/V2
Hence when osmotic pressure and temperature are the same, equal volumes of solutions would contain equal number of moles (molecules) of the solute. This relationship is analogous to Avogadro's law of gases and is referred to as Avogadro-van't Hoff law for solutions. Alternatively, this law may be stated as: Isotonic solutions at the same temperature have equimolar concentrations.
Limitations of Laws of Osmotic Pressure
Laws of osmotic pressure are not perfectly obeyed in following conditions-
1. If the solute is electrolyte.
2. If the solution is concentrate relatively.
3. If the solute is volatile.
4. If solute associates in the solution.
