Solutions and Colligative Properties - Result Question 16
22. The vapour pressure of pure benzene at a certain temperature is $640 $ $mm$ $ Hg$. A non-volatile, non-electrolyte solid weighing $2.175 g$ is added to $39.0 g$ of benzene. The vapour pressure of the solution is $600 $ $mm $ $Hg$. What is the molecular weight of the solid substance?
(1990, 3M)
Show Answer
Answer:
Correct Answer: 22. $(65.25)$
Solution:
According to Raoult’s law :
$ p =p_0 \chi_1 $
$\Rightarrow 600 =640\left(\frac{n_1}{n_1+n_2}\right) $
$\Rightarrow \frac{n_2}{n_1} =\frac{64}{60}-1=\frac{1}{15} $
$\Rightarrow n_2 =\frac{39}{78} \times \frac{1}{15}=0.033 $
$\Rightarrow \frac{2.175}{M} =0.033 $
$\Rightarrow M =65.25$
BETA
