Solutions

Vapour pressure of liquid solution

When a liquid solution is kept in a closed container, some of the molecules of liquid from its surface starts evaporating and start accumulating in the space above the surface of the liquid.

Molecules of liquid in the vapour phase move randomly and strike the liquid surface and get condensed. The process keeps on until equilibrium between evaporation and condensation reached.

The pressure exerted by molecules of vapour phase present in the vacant space of the container over the liquid surface is called vapour pressure of liquid.

Vapour pressure of liquid-liquid solution:

Figure of vapour pressure

Raoult’s Law

Francois Marte Raoult, a French chemist, establish the quantitative relation between partial pressure and mole fraction of components of liquid-liquid solution, which is known as Raoult’s Law.

Raoult’s Law states that for a solution of volatile liquids the partial vapour pressure of each component in the solution is directly proportional to its mole fraction.

Let us consider a binary solution of two volatile liquids 1 and 2.

Let partial pressure of component (1) of the solution 

And partial pressure of component (2) of the solution

Let the total vapour pressure at equilibrium

Let mole fraction of component (1) 

Let mole fraction of component (2) 

Therefore, according to Raoult’s Law

For component (1)

Similarly, for component (2)

Where are the vapour pressures of pure component (1) and (2) respectively.

Now, According to Datlton’s Law of Partial Pressure, the total vapour pressure over the solution phase in the closed container is equal to the sum of partial pressure of components of the solution.

Thus, after substituting the values of equations (i) and (ii) in equation (iii), we get

From the above equations (iv) and (v) it can be said:

(1) Total vapour pressure over the solution is related to the molar fraction of any one of the component of solution.

(2) Total vapour pressure over the solution varies linearly with the mole fraction of one of the components of solution.

(3) Total vapour pressure over the solution decreases or increases with the increase of the molar fraction of component depending on the vapour pressure of the pure component.

Graph of partial pressure p₁ or p₂ versus mole fractions x₁ and x₂ is shown in the figure.

Figure of graph

From the graph it is evident that-

(1) Line (I) and (II) are linear and passes through the points when x₁ and x₂ are equal unity respectively.

(2) Graph of p_total verses x₂ also comes linear.

(3) Depending upon the vapour pressures of the pure components, total vapour pressure over the solution may increase or decrease with increase in the mole fraction of a component.

The composition of vapour phase in equilibrium:

This is determined by the partial pressure of the components.

Using Dalton’s Law of Partial Pressure,

are the mole fractions of the component 1 and 2 respectively.

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