Solutions and Colligative Properties 1 Question 23
22. The vapour pressure of pure benzene at a certain temperature is $640 \mathrm{~mm} \mathrm{Hg}$. A non-volatile, non-electrolyte solid weighing $2.175 \mathrm{~g}$ is added to $39.0 \mathrm{~g}$ of benzene. The vapour pressure of the solution is $600 \mathrm{~mm} \mathrm{Hg}$. What is the molecular weight of the solid substance?
(1990, 3M)
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Solution:
22.5=20 \chi_{A}+70\left(1-\chi_{A}\right) & \left(\text { As } \chi_{A}+\chi_{B}=1\right) \
