Question

Calculate the mass of a non-volatile solute of molecular mass 40, which should be dissolved in 114 g octane to reduce the vapour pressure to $80\%$.

• A. 10 g
• B. 12 g
• C. 20 g
• D. 32 g

Answer 1

The correct option is A 10 g

We have the relative lowering of vapour pressure

$=\frac{P_B^0-P_t}{P_B^0}=x_A$

Percentage of lowering pressure $=\frac{P_B^0-P_t}{P_B^0}\times 100=x_A\times 100$

Moles of octane $=\frac{weight}{molecularweight}=\frac{114}{114}=1$ mole

Now.

$(100-80)\%=20\%=\frac{x}{1+x}\times 100$

$x=0.25$ moles

Mass of non-volatile solute $=0.25\times 40=10g$

Option A is correct.