Question

When acetone and chloroform are mixed together, hydrogen bonds are formed between them. Which of the following statements is correct about the solution made by mixing acetone and chloroform ?
A) On mixing acetone and chloroform will form an ideal solution
B) On mixing acetone and chloroform positive deviation is shown since the vapour pressure increases
C) On mixing acetone and chloroform negative deviation is shown since there is decrease in vapour pressure
D) At a specific composition, acetone and chloroform will form minimum boiling azeotrope

Answer 1

Solutions can be classified into two types, ideal solution and non-ideal solution, depending on their behavior. Non-ideal solutions show positive and negative deviations from Raoult’s law.

Complete answer:
Ideal solution obeys Raoult’s law over a wide range of temperature and pressures. For a binary ideal solution containing solute A and solvent A, the total vapour pressure of the solution is equal to that predicted by Raoult's law.

$${P_T} = {P_A} + {P_B} \\\ {P_T} = \left( {P_A^o \times {\chi _A}} \right) + \left( {P_B^o \times {\chi _B}} \right) \\\$$

Here, ${P_T},{P_A},{P_B}$ represents, total pressure, partial pressure of solute A in the solution and the partial pressure of solute B in the solution respectively. $P_A^o,P_B^o$ represents the vapor pressures of pure solute A and pure solvent B respectively. ${\chi _A},{\chi _B}$ represents the mole fractions of solute A and solvent B in the solution respectively.

A positive deviation from Raoult’s law is observed when the intermolecular forces of attraction between solute and solvent molecules are weaker than those present in pure solute or pure solvent. For such solutions, there is an increase in the vapour pressure.

A negative deviation from Raoult’s law is observed when the intermolecular forces of attraction between solute and solvent molecules are stronger than those present in pure solute or pure solvent. For such solutions, there is a decrease in the vapour pressure.On mixing acetone and chloroform negative deviation is shown since there is decrease in vapour pressure.

Hence, the correct answer is the option (C).

Note: Hydrogen bonds are formed when a hydrogen atom is attached to electronegative nitrogen, oxygen or fluorine atom. In chloroform, hydrogen atoms are attached to carbon atoms only. Hence, chloroform cannot form hydrogen bonds on its own. Similarly, in acetone, a hydrogen atom is attached to a carbon atom only. Hence, chloroform cannot form hydrogen bonds on its own.