NEET Chemistry Notes Solutions – Vapour Pressure
The pressure exerted by the vapours of a liquid which are in equilibrium with it at a given temperature is called vapour pressure. Vapour pressure variations with temperature is given as:
p1 and p2 are vapour pressures at T1 and T2 respectively, ∆H is heat of vaporisation.
Factors Affecting Vapour Pressure
Vapour pressure gets affected by the following factors.
- Purity of the Liquid
Pure liquid always has a vapour pressure higher than its solution.
- Nature of the Liquid
Liquids which have weak intermolecular forces are volatile and have greater vapour pressure.
The vapour pressure of a liquid increases with increase in temperature.
- Effect of Adding Solute
When a liquid contains a solute, some of the solvent molecules are replaced by the solute particles on the liquid surface and therefore, the available surface area for the escape of solvent molecule decreases.
Due to the less available area on the surface of liquid for escape, rate of evaporation and hence, the rate of condensation both lowers. The vapour pressure of liquid in solution is known as its partial vapour pressure and is less than the vapour pressure of the pure liquid at the same temperature.,
If p° be the vapour pressure of pure liquid and ps be that of liquid in solution then, lowering of vapour pressure of the liquid =p° – ps
Raoult’s law for the solutions of liquids in liquids
“The equilibrium vapour pressure of a volatile-solute is linearly proportional to the mole fraction of that component in liquid phase”.
where, A and B are volatile solute and solvent respectively. are the vapour pressures in pure state. If are the mole fraction of the components A and B respectively in the vapour phase then,
Raoult’s law for solutions of solids in liquids, i.e. for non-volatile solutes,
Limitations of Raoult’s Law
- It is applicable only to very dilute solutions.
- It is applicable only to solutions containing non-volatile and non-electrolytic solutes which exist as a single molecule.
- It is not applicable to solutes which dissociate or associate in the particular Solution
Raoult’s Law as a Special Case of Henry’s Law
According to Raoult’s law, the vapour pressure of a volatile component in a given solution is given by In the solution of a gas in a liquid, one of the components is so volatile that it exists as a gas and we have already seen that its solubility is given by Henry’s law which states that
If we compart the equations for Raoult’s law and Henry’s law, it can be seen that the partial presssure of the volatile component or gas is directly proportional to its mole fraction in the solution. Only the proportionality constant kH differs from Thus, Raoult’s law becomes a special case of Henry’s law in which kH becomes equal to