Osmosis & Osmotic pressure


⇒ The movement of solvent molecules through a semi-permeable membrane (kept between a pure solvent and its solution) from pure solvent to solution is osmosis.

⇒ The movement of solvent molecules continues till equilibrium is reached.

⇒ Solvent molecules move from lower concentration solution to higher concentration solution.


Osmotic pressure:

⇒ The excess pressure exerted on the solution side to stop the movement of solvent molecules into the solution.

⇒ Depends on concentration of solution

At a given temperature,

π α C

π = CRT

π = Osmotic Pressure

C = molarity

R = gas constant

T = Temperature

Isotonic solutions: Two solutions having same osmotic pressure at a given temperature.

Hypertonic solution:     Any solution with a higher salt concentration than normal body cells so that the water is drawn out of the cells by osmosis. Hypertonic solutions have higher osmotic pressure.

Hypotonic solution: any solution with a lower concentration than normal body cells so that water flows into the cells by osmosis. Hypotonic solutions have lower osmotic pressure.

Reverse Osmosis:

⇒ When pressure greater than osmotic pressure is applied on the solution, the solvent molecules move from solution to pure solvent. This is reverse osmosis

⇒ Used in sea water purification

⇒ Reverse osmotic pressure > osmotic pressure.

reverse osmosis

Determination of molar mass: We can determine the molecular weights of the solute using colligative properties.

The molecular weight Relates with magnitude of a colligative properties as.

Molecular Mass α 1/ Magnitude of colligative property

1. From vapour pressure:

\(\frac{\Delta {{P}_{A}}}{{{P}_{A}}}=\frac{{{w}_{B}}\times {{M}_{A}}}{{{M}_{B}}\times {{w}_{A}}}\) \({{M}_{B}}=\frac{{{P}_{A}}\times {{w}_{B}}\times {{M}_{A}}}{\Delta {{P}_{A}}\times {{w}_{A}}}\)

Where, wA = mass of solvent

MA = Molar mass of solvent

WB = mass of solute

MB = Molar mass of solute

2. From boiling point:

\({{M}_{2}}=\frac{1000\times {{w}_{2}}\times {{K}_{b}}}{\Delta {{T}_{b}}\times {{w}_{1}}}\)

M2 = molar mass of solute

W2= mass of solute

Kb = Molal elevation constant

∆Tb = elevation in boiling point

3. From freezing point:

\({{M}_{2}}=\frac{1000\times {{w}_{2}}\times {{K}_{f}}}{\Delta {{T}_{f}}\times {{w}_{1}}}\)

M2 = molar mass of solute

W2= mass of solute

Kf = Molal depression constant

∆Tf = depression in freezing point

4. From osmotic pressure:

\({{M}_{2}}=\frac{{{w}_{2}}\times R\times T}{\pi \times V}\)

M2 = molar mass of solute

R = gas constant

T = temperature

π = osmotic pressure

V = Volume of the solution in liters.