The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.

Freundlich adsorption isotherm :

Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid

adsorbent and pressure at a particular temperature. The relationship can be expressed by the following

equations :

= (n > 1).

Where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, K and n are constants.

Langmuir adsorption isotherm:

where a and b are Langmuir constant.

(i) The correct graph for Vs P according to langmuir is :