Homework Help: chemistry

Posted by robert on Saturday, October 4, 2008 at 1:14pm.

Ibuprofen, the active ingredient in Advil, is made up of carbon, hydrogen, and oxygen atoms. When a sample of ibuprofen, weighing 5.000 g, burns in oxygen, 13.86g of CO2 and 3.926 g of water are obtained. What is the simplest formula of ibuprofen?

I need to find out if I'm doing everything right here:

13.86g CO2 + 3.926g H2O

percent of C in CO2 is 12 / (12 + 2*16) = 30%
# of grams C = 13.86 * 30% = 4.16 g
moles C = 4.16 / 12 = 0.347

percent of H in H2O is 2/ (1(2) + 16) = 11.11%
# of grams H = 3.926 * 11.11% = 0.436 g
moles H = .436/1 = 0.436

# of grams O = 5.000 – (grams H + grams C) = 5.000 – 4.596 = 0.404 g
moles O = .404/16 = .0253

C = .347 / .0253 = 13.7
H = .436 / .0253 = 17.2
O = .0253 / .0253 = 1

C14H17O

I got this formula, but I looked up ibuprofen and saw that it has a C13H18O2 formula, so did I miss a doubling of oxygen moles somewhere, or maybe I'm just flat out wrong? please help!!

chemistry - DrBob222, Saturday, October 4, 2008 at 2:16pm
Two things. I don't get 30% for C in CO2 but more likle 27.27%. That will change mass C AND it will change mass O when you subtract to obtain grams O. Your H calculations look ok and the rest of the problem is ok. When I did it I obtained C13H18O2 and I looked it up on the web and found that is the formula. I don't know where you found the C14 number.
http://en.wikipedia.org/wiki/Ibuprofen
I hope this helps.
chemistry - robert, Saturday, October 4, 2008 at 4:12pm
yes that solved everything thanks!

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