Caffeine has been found to contain carbon, hydrogen, oxygen and nitrogen only. The percentage carbon was 49.48%, hydrogen was 5.19% and nitrogen was 28.85%. What is the empirical formula of the compound?
To determine the empirical formula of the compound, we will assume that we have a 100 g sample, and calculate the moles of each element.
1. Calculate the moles of carbon, hydrogen, and nitrogen.
Carbon:
49.48 g C * (1 mol C/12.01 g C) = 4.1207 moles C
Hydrogen:
5.19 g H * (1 mol H/1.01 g H) = 5.1386 moles H
Nitrogen:
28.85 g N * (1 mol N/14.01 g N) = 2.0602 moles N
2. Divide each mole value by the smallest value, to obtain the mole ratio.
Divide by the smallest mole value which is 2.0602 (moles of nitrogen):
4.1207/2.0602 = 1.9988
5.1386/2.0602 = 2.4931
2.0602/2.0602 = 1
3. Round the mole ratios (if necessary), to obtain a whole number.
C = 1.9988 ~ 2
H = 2.4931 ~ 2.5
N = 1
Since hydrogen has a 2.5 mole ratio, we need to multiply all the mole ratios by 2 to get a whole number ratio.
C = 2 * 2 = 4
H = 2.5 * 2 = 5
N = 1 * 2 = 2
Therefore, the empirical formula of this compound is C4H5N2O (oxygen was not given a percentage, so we can assume it has 1 oxygen atom in the empirical formula).