Determine the empirical formula of serotonin if it is known to be 68.2% by mass carbon, 6.86% by mass hydrogen, 15.9% by mass nitrogen, and 9.08% by mass oxygen?

Assume 100 grams and turn percentages into grams of elements.Get moles.

68.2 grams Carbon (1 mole C/12.01 grams) = 6.6786 moles C

6.86 grams hydrogen (1 mole H/1.005 grams) =  6.8056 moles H

15.9 grams nitrogen (1 mole N/14.01 grams) = 1.1349 moles N

9.08 grams oxygen (1 mole O/16.0 grams) = 0.5675 moles O

smallest mole value  is 1; divide others by that

6.6786 moles C/0.5675 moles O = 11.76---- call it 12 C

6.8056 moles H/0.5675 moles O = 11.99------cal it 12 H

1.1349 moles N/0.5675 moles O = 1.99------call it 2 N

C12H12N2O
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your empirical formula