Question

Calculate the mass of a non-volatile (Molar-mass 40 g 1 mol−

) which should be

dissolved in 114 g octane to reduce its vapour pressure to 80%.​

Answer 1

Answer:

Let p be the vapour pressure of pure octane. The vapour pressure of solution will be

100

80

p=0.8 p.

Molar mass of solute (M) and octane (m) are 40 g/mol and 114 g/mol respectively. Mass of octane, w is 114 g.

p

p−p

′

=

Mw

Wm

p

p−0.8p

=

40×114

W×114

W=8g

Hence, 8 g of solute are required.