The volume of nitrogen gas adsorbed at STP to form a monolayer on a porous solid surface is 

If the area occupied by one nitrogen gas molecule is

​then the surface area 

​of the solid is close to:

Correct option is B

Determination of surface area of adsorbent

Since the molar volume of an ideal gas at STP is 

the number of molecules N adsorbed corresponding to the volume 

Now if the area of cross-section of a single molecule is known, it can be multiplied by the above number to give the total surface area of the adsorbent.

Assuming the molecule to be spherical, its cross-sectional area can be computed as follows. If r is the radius of the molecule, it follows that

is only an approximate one since it lacks the information regarding the exact nature of packing at the surface of the adsorbent.