Question

What is modified by Raoult's Law?

Answer 1

First consider what is Raoult’s law and where can it be applied?
Raoult's law states that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution.
Mathematically it can be represented as:
$\Rightarrow {p_1} \propto {x_1}$
$\Rightarrow {p_1} = p_1^0{x_1}$
Where ${p_1}$ is the partial pressure of the first component and ${x_1}$ is the mole fraction of that component. $p_1^0$ is the vapour pressure of the pure component 1 at the same temperature.

Complete answer:
Raoult’s law is a very simple and common law that is applicable to most of the aqueous mixture and it can be used to find out the total pressure of a system having n components (volatile).
We can use the formula:
$\Rightarrow {p_i} = p_i^0{x_i}$
Total vapour pressure in the system can be found out as
$\Rightarrow {P_{Total}} = \sum\limits_o^i {{p_i}}$
As we can see this law is used particularly for separate mixtures in which each quantity is already known.
There are a few limitations of the law. Only ideal solutions obey this law perfectly. The law cannot be applied in situations where there are interactions between the components or dissociations involved in each component. All of these causes for variation in the number of solute particles present in the system
We know that Vapour pressure of a system is a colligative property and we also know that colligative properties depend on the number of particles in the system.
So we can say that the total vapour pressure obtained using Raoult’s law in systems where there is association or dissociation can give wrong values.
So the corrected formula for Raoult’s law can be given by
$\Rightarrow P{y_i} = P_i^0{\gamma _i}{x_i}$
Where, $P$ is the system pressure
${y_i}$ is the mole fraction of component $i$ in vapour phase
$P_i^0$ is the vapour pressure of pure component $i$
${\gamma _i}$ is the liquid-phase activity coefficient of component $i$
${x_i}$ is the mole fraction of component $i$ in liquid phase

Note:
When the vapour in equilibrium with a liquid mixture has a composition identical to that of the liquid, the mixture is called an azeotrope. An azeotrope can be defined as a solution whose vapour has the same composition as its liquid.
Raoult’s law is a special case of Henry’s law. The constant of proportionality is the only one that changes in both of these laws.