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Thursday, February 16, 2017

Osmosis & Osmotic pressure

Osmosis:

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.

 

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:

Where, wA = mass of solvent
MA = Molar mass of solvent
WB = mass of solute
MB = Molar mass of solute

2. From boiling point:

M2 = molar mass of solute
W2= mass of solute
Kb = Molal elevation constant
∆Tb = elevation in boiling point

3. From freezing point:

M2 = molar mass of solute
W2= mass of solute
Kf = Molal depression constant
∆Tf = depression in freezing point

4. From osmotic pressure:

M2 = molar mass of solute
R = gas constant
T = temperature
π = osmotic pressure
V = Volume of the solution in liters.

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