The elemental mass percent composition of ibuprofen (a nonsteroidal anti-inflammatory drug [NSAID]) is 75.69% C, 8.80% H, and 15.51% O. Determine the empirical formula of ibuprofen.

C13H18O2

Explanation:

We can then divide these masses by the molar masses of the elements to determine the number of moles of each element present in the 100- g  sample:

mol C = 75.69(g/C) / 12.01 (g/mol) = 6.302 mol C

mol H = 8.8g (g/H) /  1.008 (g/mol) = 8.7 mol H

mol O = 15.51 (g/O) / 16.00 (g/mol) = 0.969 mol O

The smallest mole quantity is then divided into itself and the others to give rise to a crude empirical formula:

6.302/0.969 = 6.5 C

8.7/0.969 = 9.0 H

0.969/0.969 = 1.0 O

so the crude empirical formula = C6.5H9O, Although this formula does contain the proper ratio, it does not contain only whole numbers. Because of this, we multiply each term by a common factor that will make all the numbers whole. In this case the only fractional subscript is 6.5 on carbon, so the lowest factor we can multiply to get 6.5 to a whole number is 2. The answer is C13H18O2

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