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 \[{{C}_{6}}{{H}_{6}}\] ^H^ \[=200\,\,mm\,\,Hg\] p = vapour pressure of solution = 195 mm 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 \[\frac{5}{200}=\frac{2}{m}\] \[\therefore \] \[m=80\]You need to login to perform this action.
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