Question

The vapour pressure of ether (molecular mass = 74) is 442 mm Hg at 293 K. If 3 g of a compound A are dissolved in 50 g of ether at this temperature, the vapour pressure falls to 426 mm Hg. Calculate the molecular mass of A assuming that the solution of A is very dilute.

• A. 121 $gmo{l}^{-1}$
• B. 122 $gmo{l}^{-1}$
• C. 123 $gmo{l}^{-1}$
• D. 124 $gmo{l}^{-1}$

Answer 1

Answer: (C)

123 $gmo{l}^{-1}$

we have,

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

$\frac{442-426}{442}=x_A$

$x_A$ = $\frac{n_A}{n_A+n_B}$

moles of ether = $\frac{50}{74}$ = 0.6756 moles

$x_A$ = $\frac{n_A}{0.6756}$

very dilute solution,

$n_A=0.0244$

so , molecular weight = $\frac{givenweight}{moles}$

molecular weight = 123 $gmol-1$