Solutions Question 100
Question: Which of the following liquid pairs shows a positive deviation from Raoult’s law [MP PET 1993; UPSEAT 2001; AIEEE 2004]
Options:
A) Water-nitric acid
B) Benzene-methanol
C) Water-hydrochloric acid
D) Acetone-chloroform
Show Answer
Answer:
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 Raoul’s law. In solution of methanol & benzene methanol molecules are held together due to hydrogen bonding as shown below. $ —\overset{\begin{matrix} CH_3 \\ | \\ \end{matrix}}{\mathop{O}},-H—\overset{\begin{matrix} CH_3 \\ | \\ \end{matrix}}{\mathop{O}},-H—\overset{\begin{matrix} CH_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 Raoul’s law.
