Question

State and explain Henry’s law

Answer 1

William Henry investigated the relationship between pressure and solubility of a gaseous solute in a particular solvent. According to him, “the partial pressure of the gas in the vapour phase (vapour pressure of the solute) is directly proportional to the mole fraction (x) of the gaseous solute in the solution at low concentrations.” This statement is known as Henry’s law.

Henry’s law can be expressed as,

`"P"_"solute" ∝ "x"_"solute in solution"`

`"P"_"solute" = "KH". "x"_"solute in solution"`

Here, p_solute represents the partial pressure of the gas in vapour state which is commonly called as vapour pressure. X_solute in solution represents the mole fraction of solute in the solution. nature of the gaseous solute and solvent. The above equation is a straight-line in the form of y = mx. The plot partial pressure of the gas against its mole fraction in a solution will give a straight line as shown in fig. The slope of the straight line gives the value of K_H.

Solubility of HCl gas in cyclohexane at 293 K

Limitations of Henry’s law:

1. Henry’s law is applicable at moderate temperature and pressure only.
2. Only the less soluble gases obeys Henry’s law
3. The gases reacting with the solvent do not obey Henry’s law. For example, ammonia or HCl reacts with water and hence does not obey this law.
   \[\ce{NH3 + H2O <-> NH4+ + OH-}\]
4. The gases obeying Henry’s law should not associate or dissociate while dissolving in the solvent.