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 (ΔTb)
(3) Depression of freezing (ΔTf)
(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 (ΔTb)
The elevation of boiling point of a solution is directly proportional to the molar concentration (molarity) of the solute.
ΔTb ∝ molar Concentration(m)
| ΔTb = Kb.m |
ΔTb =Elevation of boiling point, Kb=Ebullioscopic Constant or Elevation of boiling point constant
Kb for water =0.52 K kg mol-1
If m=1 then ,
| ΔTb = Kb |
Depression of freezing (ΔTf)
The depression of freezing point of a solution is directly proportional to the molar concentration (molarity) of the solute.
ΔTf ∝ molar Concentration(m)
| ΔTf= Kb.m |
ΔTf =Depression of freezing point, Kf= Molal depression constant OR Cryoscopic constant
Kf for water =1.86 K mol–1 kg–1 m= molality
If m=1 then , ΔTf= Kf
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 = Po – Ps / Po
Ra0ult’s law
Relative lowering of Vapour pressure is equal to the mole fraction of non-voltatile solute.
Po – Ps / Po = XA
Po =lowering of vapour pressure
Ps =Vapour pressure of solution
XA= 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 Kf for water is-
Ans-1.86 K mol–1 kg–1
Q Kb for water is-
Ans- 0.52 K kg mol-1
Q. Elevation of freezing point (ΔTb) =Ebullioscopic Constant or Elevation of boiling point constant
Ans-Molality(m) is equal to one.
Q.Depression of freezing (ΔTf)=Molal depression constant ( Kb)
Ans-Molality(m) is equal to one.
Q.Isontonic solution have same
Ans-Osmotic pressure
