Classical Methods:

(a) Langmuir’s model:

In adsorption systems where Langmuir’s model is followed, the maximum amount of adsorbate adsorbed per gram of adsorbent for the formation of monolayer, X_m/(mg/g), is determined. Knowing the approximate contact area of an adsorbate molecule, surface area of adsorbent could be calculated using following equation:

Example 37.1 : The volume of nitrogen gas at 1 atm and 273K required to cover 1g of the silica gel is 0.129 dm³. Calculate the surface area of the gel if each nitrogen molecule occupies an area of 16.2 X 10²⁰ m²

Solution:
Number of moles of N₂ in 0.129dm³
                                                   = 0.129/22.4 = 5.76 X 10^-3 moles of N₂ gas
Therefore, total number of N₂ ^{molecules in 0.129 dm3 of N₂}
                                                   = 5.76 x 10^-3 x 6.02 x 10²³ molecules /22.4 dm³ = 34.67 x 10²⁰
The total area of 1g of the silica gel = 34.67 x 10²⁰ x 16.2 x 10^-20 m²
                                                   = 561.74m²