Question

The vapour pressure of 2.1 %solution of a non electrolyte in water at 100 °C is 755mm Hg. Calculate molar mass of solute ​

Answer 1

Answer:

57.456 g mol-1

Explanation:

Vapour pressure = 2/1%

The vapour pressure of the pure water is required, thus let the pressure 760 mm Hg (p0).

Let the mass of water be 100 g (W1).

Mass of non-electrolyte =2.1 g (W2).

Molar mass of water (M1) =18 g

Molar mass of non-electrolyte (M2) = ??

Vapour pressure of the solution = 755 mm Hg.

By using the concept of relative lowering of vapour pressure -

(p0-p)/p0 = iX

Since it is non-electrolyte thus, i=1

(760-755)/760 = n2/(n1+n2) ≈ n2/n1 [ Since the solution is dilute (2.1%) so n2 « n1]

or, 5/760 =(W2 × M1)/(M2 × W1)

= (2.1 x 18)/(M2 × 100)

or, M2 = 57.456 g mol-1

Thus, the molar mass of solute is 57.456 g mol-1