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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jennielynn_1980 on September 23, 2011, 04:44:29 PM

Title: Determining Formula from Molecular mass?
Post by: jennielynn_1980 on September 23, 2011, 04:44:29 PM
Hi everyone.  I have figured out most of this question but the last part is really getting me.  I haven't been in a chemistry class for awhile so I am a little rusty.  If someone could tell me what direction to think in that would be great :)

A sample of 2.14 g of a gaseous organofluorine compound occupies a volume of  2.00 L at 25.5 C and 30.2 kPa.  What is the molar mass of this compound and what is it's formula?

So using PV=nRT
n= (0.298atm)(2.00L)/(0.08205 L atm K-1 mol-1)(298.65 K)
  = 0.0243 mol

Then using M=m/n
M = 2.14g/0.0243 mol
    = 87.9 g/mol

So I determined the molar mass but now how do I get the formula?

Thanks :)
Title: Re: Determining Formula from Molecular mass?
Post by: Borek on September 23, 2011, 06:54:50 PM
The only possible approach is by trial and error. Start with simple compounds (like CH3F) and see if they fit the molar mass.
Title: Re: Determining Formula from Molecular mass?
Post by: jennielynn_1980 on September 23, 2011, 07:00:06 PM
Thank you Borek.  I was trying to figure out how to do a percentage my mass setup but I wasn't getting far.