Colligative Properties 

Properties of solution which depends upon number of particles( Atom, molecules & ions) of the solute which added in it.it does not depends upon the nature or shape of particle.

There are following example of colligative properties-

(1) Osmotic pressure (π)

(2) Elevation of boiling point (ΔT_b)

(3) Depression of freezing (ΔT_f)

(4) Relative lowering of vapour pressure ( R.L.O.V.P)

Osmotic pressure (π)

Movement of solvent  from lower concentration to higher concentration through semipermeable membrane is called osmosis.

The pressure applied on solution to prevent the flow up solvent inside it is called osmotic pressure of such solution.

                                            OR

Pressure exerted by solute or solvent is called osmotic pressure .Higher the  concentration greater the osmotic pressure and vice-versa.

Mathematical expression of osmotic pressure

π = CRT

π = Osmotic pressure , C= Concentration or molarity , R = 0.0821 (L • atm/ mol • K) , T= Temperature ( in kelvin)

Reverse Osmosis ( R.O)

Movement of solvent  from higher  concentration to lower concentration through semipermeable membrane is called osmosis.

Such mechanism is used to obtained salt from sea water.

Classification of solution on the basis of osmotic pressure

(A) Hypertonic Solution

:- A solution is said to be hypertonic  with respect to another if it has a greater osmotic pressure.

(B) Hypotonic Solution

:- A solution is said to be hypotonic  with respect to another if it has a smaller osmotic pressure.

(C) Isotonic Solution

:- Isotonic solutions are those which have same osmotic pressure and Concentration.

Elevation of boiling point (ΔT_b)

The elevation of boiling point of a solution is directly proportional to the molar concentration (molarity) of the solute.

ΔT_{b ∝ molar Concentration(m)}

ΔT_b =Elevation of boiling point, K_b=Ebullioscopic Constant or Elevation of boiling point constant

K_b for water =0.52 K kg mol^-1

If m=1 then ,

Depression of freezing (ΔT_f)

The depression of freezing  point of a solution is directly proportional to the molar concentration (molarity) of the solute.

ΔT_{f ∝ molar Concentration(m)}

ΔT_f =Depression of freezing point, K_f= Molal depression constant OR Cryoscopic constant

K_f for water =1.86 K mol^–1 kg^–1   m= molality

If m=1 then ,    ΔT_f= K_f

Relative lowering of Vapour pressure ( R.L.O.V.P)

Pressure exerted by Vapour pressure on the surface of liquid is called Vapour pressure. If non-volatile solute is added in it then the Vapour pressure of it is decrease this called lowering of Vapour pressure. It is the ratio of lowering of Vapour pressure and Vapour pressure of pure solvent is called relative lowering of Vapour pressure.

R.L.O.V.P = P_o – P_s / P_o

Ra0ult’s law

Relative lowering of Vapour pressure is equal to the mole fraction of non-voltatile solute. 

P_o – P_s / P_o = X_A

P_o =lowering of vapour pressure

P_s =Vapour pressure of solution

X_A= Mole-fraction

Colligative Properties FAQ.

Q. Colligative properties depends upon

Ans-Number of particles

Q. Colligative properties does not depends upon

Ans- Nature of Solvent and shape of solvent

Q. Semipermeable only allow to flow

Ans- Only solvent molecules

Q. What is use of reverse osmosis(R.O)

Ans-Such mechanism is used to obtained salt from sea water.

Q.Relative lowering of vapour pressure is equal to mole-fraction of solute is called-

Ans-Roault’s low

Q What is unit of osmotic pressure?

Ans-atm

Q K_f for water is-

Ans-1.86 K mol^–1 kg^–1

Q K_b for water is-

Ans- 0.52 K kg mol^-1

Q. Elevation of freezing point (ΔT_b) =Ebullioscopic Constant or Elevation of boiling point constant

Ans-Molality(m) is equal to one.

Q.Depression of freezing (ΔT_f)=Molal depression constant ( K_b)

Ans-Molality(m) is equal to one.

Q.Isontonic solution have same

Ans-Osmotic pressure