Question

1.Briefly explain four colligative properties. Your explanation should contain the following:

a)definition of property

b)does it increase or decrease with concentration

c)what mathematical equation links the property to concentration

d)At least one example of natural applications of the property

Answer 1

Colligative properties are those which depends upon the quantitave aspects of the solution and not the quality aspects. The 4 colligative properties are :-

a) Relative lowering of vapor pressure

If a pure solvent has a vapor presssure of p at temperature T, then if some amount of non-volatile solute is added to the pure solvent , the vapor pressure of the solution decreases.The governing equation is :-

(P₀ - P)/P₀ = X ; where P₀ = vapor pressure of the pure solvent , P = vapor pressure of the solution after the non-volatile solute is added to it , X = mole fraction of the solution = moles of the non-volatile solute/(molaes of non-volatile solute + moles of the solvent)

2) Elevation in Boiling point

If a pure solvent has a boiling point temperature T₀, then if some amount of non-volatile solute is added to the pure solvent , the B.P of the solution increases.The governing equation is :-

T - T₀ = K_b*m ; where T = B.P of the solution , K_b = elevation constant which has different values for different solvents, m = molality = moles of the non-volatile solute/mass of the solvent in kgs

Adding salt to water increases its B.P

3) Depression in Freezing Point

If a pure solvent has a Freezing point temperature T₀, then if some amount of non-volatile solute is added to the pure solvent , the F.P of the solution decreases.The governing equation is :-

T₀ - T = K_f*m ; where T = F.P of the solution , K_f = depression constant which has different values for different solvents, m = molality = moles of the non-volatile solute/mass of the solvent in kgs

Adding glycerol to water decreases its F.P

4) Osmotic Pressure

It is used in purifying sea water

The governing equation is :-

P = C*R*T

where P = pressure of the solution exerted , C = molar concentration of the solute , R = Universal Gas Constant, T = temperature in kelvin

Increase in the concentration of the non-volatile solute increases the osmotic pressure.