A carbon compound contains 12.8% of carbon, 2.1% of hydrogen and 85.1% of bromine. The molecular weight of the compound is 187.9. What is the molecular formula of the compound? (Atomic weight: H = 1.008, C = 12.0, Br = 79.9)

1 Answer
Sep 11, 2017

Answer:

#"Molecular formula"# #-=# #C_2H_4Br_2#

Explanation:

As always with these problems, it is useful to assume #100*g# of unknown compound....and thus find an empirical formula...

#"Moles of carbon"=(12.8*g)/(12.011*g*mol^-1)=1.066*mol.#

#"Moles of hydrogen"=(2.1*g)/(1.00794*g*mol^-1)=2.084*mol.#

#"Moles of bromine"=(85.1*g)/(79.90*g*mol^-1)=1.066*mol.#

And we divide thru by the LOWEST molar quantity to give an empirical formula of.....

#CH_2Br#

But we gots a molecular mass, and we know that the molecular formula is a whole number multiple of the empirical formula.

And so #187.9*g*mol^-1=nxx(12.011+2xx1.00794+79.9)*g*mol^-1#

And thus #n=2#, and the molecular formula is #C_2H_4Br_2#...

I prefer questions that quote actual microanalytical data. #C_2H_4Br_2# is a liquid, and you would rarely be able to get combustion data on a liquid (the analyst would probably laugh at you!).