Based on the osmatic pressure, the solutions are classified into three major types: isotonic, hypertonic and hypotonic solutions.
i. Isotonic solutions :
When two solutions made of different solutes have same osmotic pressure, they are said to be isotonic or iso-osmotic solutions.
The solution is represented as π1 = π2 where, π1
and π2 represents osmotic pressures of solutions.
Example: Solutions of urea and glucose (that do not dissociate in water); have same
concentration (c1 = c2) besides osmotic pressure.
Note: Recall the relation among osmotic pressure and concentration (in molarity) as given
Osmotic pressure (π) = cRT
where c = concentration measured in molarity and is given by
As concentration is also related to the mole (n) and volume (v) as c = n / v;
for isotonic solutions, c1 = c2
⇒ [n1] / v1 = [n2] / v2
Since, mole is the ratio of weight(w) of substance to its molecular weight (m),
therefore, w1 / m1 × v1 = w2 /
m2 × v2
There are isotonic solutions like urea + NaCl and urea + benzoic acid that have
π1 = π2 but c1 ≠ c2
ii. Hypertonic and hypotonic solutions
In a mixture, the solution that has more osomotic pressure is said to be hypertonic and the
one with lower is called hypo tonic. In other words, hyper tonic solution has high
concentration of solute. When the mixture is placed in a membrane tank, flow of solvent is
always observed from the hypotonic.
Example: Mixture of 1 N H2SO4 and 1 N HCl having osmotic pressures
π1 and π2 respectively.
Ionization of H2SO4:
Total no of particles = 3
No of H+ particles = 2
Therefore, c = 3/2 = 1.5 and π1 = 1.5 RT
Ionization of HCl:
Total no of particles = 2
No of H+ particles = 1
Therefore, c= 2/1 = 2 and π2 = 2 RT
Therefore, π1 <π2
As the osmotic pressure is more for HCl, it is hypertonic
In the solution, size of solvent (water) is smaller than solute (acids:
HCl,H2SO4). This makes water move from hypertonic solution (HCl) to
less concentrated H2SO4 solution by means of membrane.