A) Water-nitric acid
B) Benzene-methanol
C) Water-hydrochloric acid
D) Acetone-chloroform
Correct Answer: B
Solution :
In solution showing positive type of deviation the partial pressure of each component of solution is greater than the vapour pressure as expected according to Raoult?s law. In solution of methanol & benzene methanol molecules are held together due to hydrogen bonding as shown below. \[---\overset{\begin{matrix} C{{H}_{3}} \\ | \\ \end{matrix}}{\mathop{O}}\,-H---\overset{\begin{matrix} C{{H}_{3}} \\ | \\ \end{matrix}}{\mathop{O}}\,-H---\overset{\begin{matrix} C{{H}_{3}} \\ | \\ \end{matrix}}{\mathop{O}}\,-H---\] On adding benzene, the benzene molecules get in between the molecule of methanol thus breaking the hydrogen bonds. As the resulting solution has weak intermolecular attraction, the escaping tendency of alcohol & benzene molecule from the solution increases. Consequently the vapour pressure of the solution is greater than the vapour pressure as expected from Raoult?s law.
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