Question

calculate the mass of a nonvoltile solute (molar mass 40gm/mol) which should be disolve in 114gm octane reduce its vapour pressure to 80%
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Answer 1

Answer :

Let the vapour pressure of pure octane be p10.

Then, the vapour pressure of the octane after dissolving the non-volatile solute is 80/100 p10 = 0.8 p10.

• Molar mass of solute, M2 = 40 g mol - 1

• Mass of octane, w1 = 114 g

Molar mass of octane, (C8H18), M1 = 8 × 12 + 18 × 1 = 114 g mol - 1

Applying the relation,

=> (p10 - p1) / p10 = (w2 x M1 ) / (M2 x w1 )

=> (p10 - 0.8 p10) / p10 = (w2 x 114 ) / (40 x 114 )

=> 0.2 p10 / p10 = w2 / 40

=> 0.2 = w2 / 40

=> w2 = 8 g

Hence, the required mass of the solute is 8 g.

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