• question_answer 19) The vapour pressure of benzene at a certain temperature $640\text{ }mm$ of Hg. A non-volatile and non-electrolyte solid weighing $2.175\text{ }g$ is added to $39.08\text{ }g$ of benzene. If the vapour pressure of the solution is $600mm$ of Hg, what is the molecular weight of solid substance?                 A)  $49.50$                 B)  $59.60$C)  $69.60$                 D)  $79.87$

$\frac{{{P}^{o}}-{{P}_{S}}}{{{P}^{o}}}=\frac{w/m}{W/M};(640-600)/640=wM/mW$ $40/640=2.175\times 78/m\times 39.08$ $m=2.175\times 78\times 640/39.08\times 40=69.458\cong 69.60$