A) 20
B) 40
C) 60
D) 80
Correct Answer: D
Solution :
Key Idea: Use Raoults law to find molecular mass of solute. Given,\[{{p}^{o}}=\] Vapour pressure of \[{{C}_{6}}{{H}_{6}}\] \[=200\text{ }mm\text{ }Hg\] \[p=\]vapour pressure of solution \[=195\text{ }mm\text{ }Hg\] \[W=\]weight of solvent \[({{C}_{6}}{{H}_{6}})=78\,g\] \[M=\]molecular wt. of \[{{C}_{6}}{{H}_{6}}=78\] \[w=\]weight of solute \[=2\,g\] \[m=\]molecular wt. of solute = ? \[\frac{{{p}^{o}}-p}{{{p}^{o}}}=\frac{w\times M}{m\times W}\] or \[\frac{200-195}{200}=\frac{2\times 78}{m\times 78}\] or \[\frac{5}{200}=\frac{2}{m}\] \[\therefore \] \[m=80\]
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