## Saturday, February 2, 2008

### IIT JEE Revision Ch 9. SOLUTIONS - Core Points

Jee Syllabus

Solutions:
Raoult's law;
Molecular weight determination from lowering of vapor pressure,
Molecular weight determination from elevation of boiling point
Molecular weight determination from depression of freezing point.
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Raolt's Law

In the case of a solution of two liquids, A and B, the total vapor pressure Ptot(P total) above the solution is equal to the sum of the vapor pressures of the two components, PA and PB and

PA = PA° * Am
PB = PB° * Bm

Where
PA° = vapour pressure created by 1 mol of liquid A
Am = mole fraction of liquid A in the solution
PB° = vapour pressure created by 1 mol of liquid A
Bm = mole fraction of liquid A in the solution

The pressure exerted by the vapours above the liquid surface in equilibrium with the liquid at a given temperature is called vapour pressure.

If a small amount of non-volatile solute is added to the the solvent, the vapour pressure of the solution becomes less than that of the pure solvent.

Some properties of the solution depend only on the number of solute particles but on the nature of the solute. These are called colligative properties or democratic properties.

The four important ones are:
i) Relative lowering in vapour pressure
ii) elevation in boiling point
iii) depression in freezing point
iv) osmotic pressure

Molecular weight determination from lowering of vapor pressure
Molar mass of a solute can be found from the property of lowering of vapor pressure of a solution.

Mb = (Wb*Ma)/[Wa*(Pa°-Pa)/Pa°]

Wb = weight of solute particles, Wa= weight of solvent
(Pa°-Pa)/Pa° = decrease in vapour pressure of solution
Ma = Molar mass of solvent

Molecular weight determination from elevation of boiling point

Mb = [Kb*Wb*1000]/[ΔTb*Wa]
ΔTb = increase in boiling point of the solution after adding the solute
Kb = molal elevation constant or ebulloscopic constant
= the elevation in boiling point for 1 molal solution, i.e., a solution containing 1 gram mole of solute dissolved in 1000 g of the solvent.

Molecular weight determination from depression of freezing point.

when a non-volatile solute is added to a solvent, the freezing point of the solution is always lower than that of the pure solvent.

The depression in freezing temperature is proportional to the molal concentration of the solution.
ΔTf α m Or ΔTf = Kf*m

Kf is the molal depression constant. also called molal cryoscopic constant. It is defined as the depression in freezing point for 1 molal solution i.e., a solution containing 1 gram mole of solute dissolved in 1000 g of solvent.

Mb = [Kf*Wb*1000]/[ΔTf * Wa]