A) 2.04
B) 1.05
C) 0.03
D) 0.09
Correct Answer: C
Solution :
[c] Loss in mass of solution =0.78 g Loss in mass of solvent =0.02 g \[\Rightarrow \frac{{{P}_{o}}-{{P}_{S}}}{{{P}_{S}}}=\frac{0.02}{0.78}=\frac{\text{Loss}\,\,\text{in}\,\,\text{mass}\,\,\text{of}\,\,\text{solvent}}{\text{Loss}\,\,\text{in}\,\,\text{mass}\,\,\text{of}\,\,\text{solution}}\]Let \[A\to solvent\,\,\,\,B\to Solute\] \[\therefore \frac{{{P}_{o}}-{{P}_{S}}}{{{P}_{S}}}=\frac{{{n}_{B}}}{{{n}_{A}}}=\frac{{{w}_{B}}}{{{M}_{B}}}\times \frac{{{M}_{A}}}{{{W}_{A}}}\] \[\frac{0.02}{0.78}=\frac{5}{{{M}_{B}}}\times \frac{18}{100}\] \[\Rightarrow {{M}_{B}}=35.1g\approx 35g\] Relative lowering of \[VP=\frac{{{P}_{o}}-{{P}_{S}}}{{{P}_{o}}}={{X}_{B}}\] \[\therefore {{n}_{B}}=\frac{5}{35}=\frac{1}{7},{{n}_{A}}=\frac{100}{18}\] \[{{X}_{B}}=\frac{\frac{1}{7}}{\frac{1}{7}+\frac{100}{18}}=\frac{18}{(18+700)}=0.0251\cong 0.03\]
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