Q. 24

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Answer :

(a) Given:

Mass Of Solute (W1) = 5.0 g

Mass of solvent (W2) = 95g

Vapour pressure of solution (p)=23.375mm Hg

Vapour pressure of pure water (P° = 23.75 mm Hg)

Molar mass of solute (M1) = ?

Acc to Raoults Law


23.375= 23.75 × A

𝒳 A == 0.98

0.02 = = = = =


M1= 46.42 g/mol


(i) Colligative properties are those properties which depend on the number or amount of solute particles irrespective of its nature. As we know Osmotic pressure is also directly proportional to the concentration f the solution, so it also depends on the no of solute particle and hence is a colligative property. Mathematically,

(Where π is osmotic pressure)

(ii) Molality is no of moles of solute dissolved in 1 kg of solvent whereas molarity is no of moles of solute dissolved in 1 Litre of solution. As Molality is mass dependent which is independent of temperature and pressure conditions in comparison with volume which is a function of pressure and temperature, Molality is preferred to express concentration.


(a) If CaCl2 is completely disassociated so, it will form 3 species.

CaCl2 Ca2+ + 2 Cl-

So, i=3

Acc to formula,

ΔTf = i Kf m

Where Kf is molal freezing point

i is vant hoff factor

Δ Tf is change in freezing point

m is molality


= 19.89 gm.

(b) (i)

Blood contain NaCl which makes it saline. If blood cells are placed in distilled water, the water will move from higher concentration to lower concentration, and hence will move inside the cell, making it swell which may even burst.

(ii) When chloroform is added to acetone the temperature rises due to release of heat as it is an exothermic process. When chloroform is added to acetone new hydrogen bonding take place which makes the molecule more stable, and when there is a formation of stable bond, a high amount of energy is release, rising its temperature.

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