Q17 of 50 Page 1

Calculate the mass of a non-volatile solute (molar mass40 g/mol) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.

Given:

Molecular Mass of Solute, M2 = 40g


The weight of solvent, W1 = 114g


Let vapour pressure of pure octane be P1°. Thus the vapour pressure of octane after dissolving the solute is given by [80/100] × P1° = 0.8 P1°.


Molecular Mass of Octane [C8H18], M1 = [12× 8] + [1× 18]


= 96 + 18


= 114g


To find: Mass of Solute to be dissolved, W2


Solution:


By Raoult’s Law we have:





W2 = 8g


Therefore the mass of solute to be dissolved in octane is 8g.


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