[CHM 111] Molecular Formula Question

[Andrew J. Pounds]

On 10/17/20 2:24 PM,  wrote:
> Dear Dr. Pounds,
> To find the molecular formula, we are supposed to divide the molar
> mass of the substance by the molar mass of the empirical formula.
> However, on the report it asks us to put the molar mass based on the
> molecular formula. So, I am a little confused. Do we use the first
> molar mass to find the molecular formula and then find the molar mass
> again to compare the two or do we use something else to find the
> molecular formula? I tried asking some of the people in the class, but
> they were also a little confused. 
>
> Sincerely, 
>
>
Use the method from page 115-116 to find the empirical formula.  Your
empirical formula will have a mass - calculate it.  Then based on the
average molar mass that you determined experimentally and the empirical
mass that you determined from the elemental analysis, figure out how
many "empirical masses" you have.


Lets imagine that from my elemental analysis I determined that my
empirical formula was $\mathrm{C_3H_6O}$, which has a mass of 58
g/mol.   I my determined molar mass from the experiment was close to 60,
then I would conclude that $\mathrm{C_3H_6O}$ was the molecular
formula.  If my determined molar mass from the experiment was closer to
120, then I would conclude that the molecular formula was
$\mathrm{C_6H_{12}O_2}$.  If the determined molar mass from the
experiment was closer to 180, then I would conclude that the molecular
formula was $\mathrm{C_9H_{18}O_3}$.  



-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University,  Macon, GA 31207   (478) 301-5627

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