Calculation of Vapour Pressure by Raoult’s Law

When a pan of water is heated with the lid on, minutes later, the cover clatters, and heated water spills onto the burner. How does this happen? When the water is warmed, the atoms gain enough kinetic energy to beat the forces that keep them in the water and escape in the form of vapour. They create a population of particles in the vapour stage over the fluid in the vessel, which creates vapour pressure. In this example, the water created sufficient pressure to lift the pan’s lid and escape the vapour. If a tightly sealed jar contains vapour, and the vapour pressure rises, the jar may explode. In this segment, we go over vapour pressure in more depth and tell the best way to calculate vapour pressure.

What is the vapour pressure formula?

When a solid dissolves in a liquid, a solution is formed. The addition of the solute lowers the vapour pressure formed by this solution. Here is the vapour pressure formula using Raoult’s Law, which explains how adding a solute changes the vapour pressure of a liquid solution.

Psolution = (Xsolvent) ( Posolvent )

Xsolvent = the solvent’s mole fraction in the solution

Psolution = the solution’s vapour pressure

Posolvent = the solvent’s vapour pressure

How do you calculate a solution’s vapour pressure?

Raoult’s Law is used to accomplish this, which states that the component’s partial vapour pressure is equal to its vapour pressure when pure multiplied by the mole fraction of that component in an ideal solution.

Raoult’s Law:

According to the Law, the partial pressure is directly proportional to the mole fraction of the solute component. So, according to Raoult’s Law, the partial pressure of A will be

PA ∝ XA

PA = PA0 XA

PA0 denotes the vapour pressure of pure liquid component A. 

the partial pressure of B will be the same.

PB ∝ XB

PB = PB0 XB

Where PB0 represents the vapour pressure of pure liquid components, B. Dalton’s Law of partial pressures will now be applied. This Law states that the total pressure (Ptotal) of a solution in a container equals the sum of its constituent partial pressures. This is the case.

Ptotal = PA + PB

Ptotal  = PA0 XA + PB0 XB

Also, because XA + XB = 1, we can write the relationship as follows:

Ptotal = PA0 + (PB0 – PA0) XB

Mole fraction:

The mole fraction refers to the ratio of solvent moles to total moles in the solution.

While working with a volatile solute in a  solution, the vapour pressure of that solution is

P(solution) = X(solvent) . P° solvent + X(solute) . P° solute

Where,

 P° solute= the vapour pressure of the pure solute

X(solute) = the mole fraction of the solute.

The above equation allows us to draw the following conclusions:

• The above equation is a straight line with a slope of (P2o-P1o) and a y-intercept of P1o between Ptotal and X2.
• The mole fraction of component 2 determines the total vapour pressure present above a solution.
• The vapour pressure of components 1 and 2 in their pure states and the mole fraction of component 2 in the solution determines the total vapour pressure above a solution.

Vapour pressure characteristics

• Compared to a liquid solution, a pure liquid has more vapour pressure. 
• It is inversely proportional to the attraction forces that exist between liquid molecules.
• As the temperature rises, so does it. This is due to the molecules gaining kinetic energy and rapidly vaporising.

Liquid-Liquid Solutions vapour pressure

We’ll look at two volatile liquid solutions to see how they stack up. Let’s call them A and B. After placing the volatile liquid and its constituents in a closed vessel, we discover that equilibrium is established between the liquid and vapour phases. Assume Ptotal is the total vapour pressure in equilibrium, and PA and PB are components A and B’s partial vapour pressures. Furthermore, the respective mole fractions are xA and xB. Raoult’s Law is used to determine the vapour pressure of a volatile liquid.

Boiling point

The temperature at which the vapour pressure at the surface of a liquid becomes equal to the pressure exerted by the surroundings is called the boiling point of the liquid.

Conclusion

The pressure formed by the vapour of a liquid (or solid) over the liquid’s surface is known as vapour pressure. This pressure is formed in a thermodynamic equilibrium state in a closed container at a specific temperature. The equilibrium vapour pressure determines the rate of evaporation of liquid. The temperature raises the vapour pressure. The boiling point of a liquid is defined as the point at which the pressure exerted by the surrounding environment equals the pressure exerted by the vapour. This chapter will learn more about vapour pressure on Liquids and their properties.