A) 32
B) 16
C) 64
D) 48
Correct Answer: B
Solution :
Step I Calculation of number of moles \[{{p}_{solution}}={{p}_{benzene}}\times {{\chi }_{benzene}}\] \[{{p}_{solution}}=0.8\,bar\] \[{{p}_{{{C}_{6}}{{H}_{6}}}}=1\,bar\] \[{{\chi }_{{{C}_{6}}{{H}_{6}}}}=\frac{{{p}_{solution}}}{{{p}_{{{C}_{6}}{{H}_{6}}}}}=\frac{0.8\,bar}{1\,bar}\] \[{{\chi }_{{{C}_{6}}{{H}_{6}}}}=0.8\] \[{{\chi }_{{{C}_{6}}{{H}_{6}}}}=\frac{Moles\,of\,{{C}_{6}}{{H}_{6}}}{Moles\,of\,{{C}_{6}}{{H}_{6}}+Moles\,of\,solute}\] \[{{\chi }_{{{C}_{6}}{{H}_{6}}}}=\frac{(39g)/78\,g\,mo{{l}^{-1}}}{(39\,g)(78mo{{l}^{-1}})+{{n}_{solute}}}\] \[0.8=\frac{0.5}{0.5+{{n}_{solute}}}\] \[0.4+0.8n\,solute=0.5\] \[0.8n\,solute=0.5-0.4\] \[n\,solute=0.1/0.8\] \[n\,solute=\frac{1}{8}mol\] Step II Calculation of molecular mass of solute \[Molecular\text{ }mass=\frac{Mass\text{ }of\text{ }solute}{Number\text{ }of\text{ }moles\text{ }Of\text{ }solute}\] \[Molecular\text{ }mass=\frac{2g}{(1/8)mol}\] \[=\text{ }16\text{ }g\text{ }mo{{l}^{-1}}\]
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