Posted by Chris on .
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, 13.86 g of CO2 and 3.926 g of water are obtained. What is the simplest formula of ibuprofen?
My work:
13.86 g CO2/ 12.01 g C= 1.15 g C
3.926 g H2O/ 2.02 = 1.94 g H
13.86 g CO2 + 3.926 g H2O = 17.48sub6 g products
17.48sub6 g  3.09 = 14.40 g O
1.15 g C x 1mol/12.01 g= .0957sub5 mol C
1.94 g H x 1mol/1.01 g= 1.92sub0 mol H
14.40 g O x 1mol/16.00= .9000 mol O
divide all by .0957sub5...
the formula comes out to C10H201O94
My problem is, the 14.40 g O isn't just the oxygen in the ibuprofen, but the oxygen in the combustion as well. how do I find out just what's in the ibuprofen so I get the correct formula?
Your work is wrong.
To get the grams of C in CO2, find the fraction of C in CO2, which is 12/(12+2*16)= 12/40
Then, grams C= 13.86 * 12/40 g C
then moles C= gramsC/12
Do the same with H.
Then, to find O, add the grams H and grams C, subtract them from 5.000. Then figure the grams O, and moles O.
Now, having moles of all, do this.
moles C xxxx
moles H yyyy
moles O zzzz
take the smallest of the three numbers, and divide it into all the numbers to get a whole number ratio.
example
moles C 4.53
moles H 9.05
moles O 12.22
divide all by 4.53
moles C 1
moles H 2
moles O 3
CH2O3 in this example. Here is another more complicated:
moles C 4.53
moles H 11.35
moles O 12.22
divide,
moles C 1
moles H 2.5
moles O 3
Now here, to get a whole number ratio , double the numbers..
moles C 2
moles H 5
moles O 6
C2H5O6
Thank you, I assumed something was wrong when I didn't end up using the 5.000 g in my calculation.
I've used this setup before, and for this problem after doing it this way, I received C101H70O20. This doesn't seem right to me. I think any error I have might have occurred around here:
moles C= .3149sub 2
moles H= .217sub 8
moles O= .0623sub 6
I divided everything by .0623sub 6 [smallest number] and got
moles C= 5.05
moles H= 3.5
moles O= 1
and multiplied everything by 20 to get a whole number ratio.
The molar masses I used for all my calculations:
12.01 g/mol = C
1.01 g/mol = H
16.00 g/mol = O
Did I make an error, or is this correct?
Thank you, I assumed something was wrong when I didn't end up using the 5.000 g in my calculation.
I've used this setup before, and for this problem after doing it this way, I received C101H70O20. This doesn't seem right to me. I think any error I have might have occurred around here:
moles C= .3149sub 2 OK to here,
moles H= .217sub 8 There are 2 mols H in H2O. This will change mols H as well as grams H AND grams oxygen.
moles O= .0623sub 6
I divided everything by .0623sub 6 [smallest number] and got
moles C= 5.05
moles H= 3.5
moles O= 1
and multiplied everything by 20 to get a whole number ratio.
The molar masses I used for all my calculations:
12.01 g/mol = C
1.01 g/mol = H
16.00 g/mol = O
Did I make an error, or is this correct?
See comments at H line.

chemistry 
robert,
im getting that the moles C = .347
moles H = .436
moles O = 0.025
and getting a formula of C14H17O
However, i looked it up on wiki and saw that the formula for ibuprofen is in fact i C13H18O2.
the C/H discrepancy is understandable, but the O2 to O is not. Could i have missed something that leads to a doubling of the moles O?