how should we calculate molecular formula when vapour density does not given?

To find the no. of moles use the formula:

n=m/MM or moles= mass divided by Molecular mass

At STP conditions , we have many relations ,

     If Volume is known at stp for gas.
     Mole = Volume / 22.4 liters

     If no. of molecules/ions/atoms... are known
     Mole = no. of substance / (6.023 * 10^23)

     If mass , and formula mass are known
     Mole = Mass / Formula Mass

     If vapour density and mass are known
     Mole = Mass / (Vapour Density * 2)            etc.

If u r question is to calculate no. of moles if vapour density is given, then

molecular formula : A chemical formula that shows the number and kinds of atoms in a molecule.

MM = dRT/P
I would like to explain through an example:
Q) A certain compound containing only carbon and hydrogen was found to have a vapor density of 2.550 g/L at 100°C and 760 mm Hg. If the    empirical formula of this compound is CH, what is the molecular formula of this compound?

Ans) use the formula: MM = dRT/P
100°C = 373K
760 torr = 1atm
MM = (2.550g/L)(0.08206/mol*K)(373K)/(1atm)
MM = 78.05g/mol
MM CH = 13 g/mol
78.05g/mol / 13g/mol = 6

compund is :C6H6

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Naga Ramesh
IIT Kgp - 2005 batch