[CHM 112] Lab Question

[Andrew J. Pounds]

On 06/25/13 19:35,  wrote:
> Hey Dr. Pounds,
> I'm working on my calculations for lab. I was able to find the 
> empirical formula with ease, but I'm not sure how to go about finding 
> the molecular formula. I have added up the formula mass, but won't I 
> need a molecular mass to compare it to? I only have the average molar 
> mass that I found via experiments.

That is correct -- you have a MOLAR mass that you determined from your 
experiment today (using the mass of the vapor divided by the moles of 
vapor determined from $n=\frac{PV}{RT}$).  Now, lets for the sake of 
argument let's imaging that your empirical formula (detemined from the 
elemental analysis) is $\mathrm{CH_4}$ and that your molar mass from the 
expement was 33 g/mol.   The molar mass of $\mathrm{CH_4}$ is 16 g/mol 
-- so your MOLECULAR formula would be $\mathrm{C_2H_8}$ because this is 
the molecular formula that is an whole number multiple of the empirical 
formula that is has a molecular mass (32 g/mol) close to your 
experimentally determined molecular mass.


-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627
http://faculty.mercer.edu/pounds_aj

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