I see a few issues here. Let me pose this as a study exercise ...

* What do you really want to obtain in ChemDraw? The 3D size of the molecule. This has nothing to do with calculating the active surface area. It only has to do with defining the size of the molecule.

* How do molecules absorb on surfaces? They absorb as a 3D molecule on to a 2D surface area.

* How do you obtain the 2D surface area that is occupied by the 3D molecule that you model in ChemDraw? You project the molecule as a "cylinder" to sit on a a flat plane.

* What is the proper cylinder projection of the 3D molecule? It depends on the surface and the molecule. For example, an amine-silane could absorb in one case with the amine end down or in another case with the siloxane end down.

* How do I find out which is the proper bonding geometry for my molecule? You research this in the literature.

* How does this relate to the coverage of a molecule on a surface? Once you know the projected area per molecule Am (m2/molec), you can determine the coverage of the molecules on a surface area A (m2) approximately as Nm (molecules) = A / Am.

* How does this fit to the Langmuir isotherm? That is an exercise left to the reader.

Dear Sir: The specific surface area of any adsorbent was determined by the method of methylene blue (MB) described by Hang and Brindly [Clays Clay Miner. 1970; 18: 203- 212.] .  The adsorption capacity of MB was obtained from batch adsorption experiments.

The value of qmax can be obtained the Langmuir isotherm. Depending on this value the specific surface area (S) of the adsorbent can be calculated from the equation:

                                  S=qmax ×CSA×N (4)

where CSA is the cross-sectional area occupied by MB molecule (130 °A2) and NA is the Avogadro's number (mol−1).

you must have data or design several runs for a experiment to fit the data to the Langmuir isotherm, the you find slope or intersection your desired constant values to calculate your targets.