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April 29, 2016
Posted by **Chris** on Tuesday, October 17, 2006 at 5:12pm.

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 set-up 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 set-up 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?

- chemistry -
**robert**, Saturday, October 4, 2008 at 1:10pmim 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?