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

7 years ago


Answers : (1)


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


Please feel free to post as many doubts on our disucssion forum as you can. If you find any question difficult to understand - post it here and

we will get you the answer and detailed solution very quickly.We are all IITians and here to help you in your IIT JEE preparation. All the best.


Naga Ramesh

IIT Kgp - 2005 batch


7 years ago

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