Answer
$C_{13}H_{18}O_{2}$
Work Step by Step
In the 100g sample of ibuprofen, 75.69g carbon, 8.80g hydrogen and 15.51g oxygen are present.
Moles of C=$\frac{75.69g}{12.01g/mol}=6.302\,mol$
Moles of H=$\frac{8.80g}{1.008g/mol}=8.73\,mol$
Moles of O=$\frac{15.51g}{15.999g/mol}=0.9694\,mol$
Since 0.9694 is the smallest value, division by it gives a ratio of 6.5:9:1 for C:H:O.
Multiplying by 2, we get the ratio in whole numbers.
That is, the ratio is 13:18:2 for C:H:O.
We obtain the empirical formula by mentioning the numbers in the ratio after writing the respective symbols.
Thus, the empirical formula is $C_{13}H_{18}O_{2}$
