Question 14 Marks
Read the following text carefully and answer the questions that follow:
Raoult's law for volatile liquids states that the partial vapour pressure of each component in the solution is directly proportional to its mole fraction, whereas for a non $-$ volatile solute, it states that the vapour pressure of a solution of a non $-$ volatile solute is equal to the vapour pressure of the pure solvent at that temperature multiplied by its mole fraction. Two liquids $A$ and $B$ are mixed with each other to form a solution, the vapour phase consists of both components of the solution. Once the components in the solution have reached equilibrium, the total vapour pressure of the solution can be determined by combining Raoult's law with Dalton's law of partial pressures. If a non $-$ volatile solute $B$ is dissolved into a solvent $A$ to form a solution, the vapour pressure of the solution will be lower than that of the pure solvent. The solutions which obey Raoult's law over the entire range of concentration are ideal solutions, whereas the solutions for which vapour pressure is either higher or lowerthan that predicted by Raoult's law are called non $-$ ideal solutions. Non $-$ ideal solutions are identified by determining the strength of the intermolecular forces between the different molecules in that particular solution.They can either show positive or negative deviation from Raoult's law depending on whether the $A - B$ interactions in solution are stronger or weaker than $A - A$ and $B - B$ interactions.
$i. 20 \ mL$ of a liquid $A$ was mixed with $20 \ mL$ of liquid $B$. The volume of resulting solution was found to be less than $40 \ mL$. What do you conclude from the above data?
$ii$. Which of the following show positive deviation from Raoult's law? Carbon disulphide and Acetone; Phenol and Aniline; Ethanol and Acetone.
$iii$. The vapour pressure of a solution of glucose in water is $750\ mm\ Hg$ at $100^\circ C$. Calculate the mole fraction of solute.
$($Vapour pressure of water at $373 K = 760 \ mm\ Hg)$
OR
$iii$. The boiling point of solution increases when $1$ mol of $\ce{NaCl}$ is added to $1$ litre of water while addition of $1$ mol of methanol to one litre of water decreases its boiling point. Explain the above observations.
Raoult's law for volatile liquids states that the partial vapour pressure of each component in the solution is directly proportional to its mole fraction, whereas for a non $-$ volatile solute, it states that the vapour pressure of a solution of a non $-$ volatile solute is equal to the vapour pressure of the pure solvent at that temperature multiplied by its mole fraction. Two liquids $A$ and $B$ are mixed with each other to form a solution, the vapour phase consists of both components of the solution. Once the components in the solution have reached equilibrium, the total vapour pressure of the solution can be determined by combining Raoult's law with Dalton's law of partial pressures. If a non $-$ volatile solute $B$ is dissolved into a solvent $A$ to form a solution, the vapour pressure of the solution will be lower than that of the pure solvent. The solutions which obey Raoult's law over the entire range of concentration are ideal solutions, whereas the solutions for which vapour pressure is either higher or lowerthan that predicted by Raoult's law are called non $-$ ideal solutions. Non $-$ ideal solutions are identified by determining the strength of the intermolecular forces between the different molecules in that particular solution.They can either show positive or negative deviation from Raoult's law depending on whether the $A - B$ interactions in solution are stronger or weaker than $A - A$ and $B - B$ interactions.
$i. 20 \ mL$ of a liquid $A$ was mixed with $20 \ mL$ of liquid $B$. The volume of resulting solution was found to be less than $40 \ mL$. What do you conclude from the above data?
$ii$. Which of the following show positive deviation from Raoult's law? Carbon disulphide and Acetone; Phenol and Aniline; Ethanol and Acetone.
$iii$. The vapour pressure of a solution of glucose in water is $750\ mm\ Hg$ at $100^\circ C$. Calculate the mole fraction of solute.
$($Vapour pressure of water at $373 K = 760 \ mm\ Hg)$
OR
$iii$. The boiling point of solution increases when $1$ mol of $\ce{NaCl}$ is added to $1$ litre of water while addition of $1$ mol of methanol to one litre of water decreases its boiling point. Explain the above observations.
Answer
View full question & answer→$i$. Solution shows a negative deviation from Raoult's law
$A-A $ and $B-B$ interactions are weaker than $A-B$ interactions.
$ii.$ Carbon disulphide and acetone, Ethanol and acetone.
$iii.$ According to Raoult's law:
$P_1=p_1^0 x_1 $ or $ x_1=\frac{p_1}{p_1^0}$
$x_1=\frac{750}{760}=0.987$
$x_2=1-x_1$
$=1-0.987=0.013$
$A-A $ and $B-B$ interactions are weaker than $A-B$ interactions.
$ii.$ Carbon disulphide and acetone, Ethanol and acetone.
$iii.$ According to Raoult's law:
$P_1=p_1^0 x_1 $ or $ x_1=\frac{p_1}{p_1^0}$
$x_1=\frac{750}{760}=0.987$
$x_2=1-x_1$
$=1-0.987=0.013$