# Question #f1fda

Oct 7, 2014

In order to determine the empirical formula from percent composition, you need to convert the percentage of each element to grams, and then to moles. In this case, since the percentages add up to 100%, you can directly convert each percentage to grams. To get the number of moles for each element, you divide its mass in the sample by its molar mass from the periodic table (atomic weight in grams).

C = 37.51g
H = 4.20g
O = 58.29g

Convert masses to moles.

C = 37.51g/12.011g/mol = 3.123 mol C
H = 4.20g/1.008g/mol = 4.167 mol H
O = 58.29/15.999g/mol = 3.643 mol O

Divide the number of moles for each element by the lowest number of moles to get the lowest whole number ratio.

C = 3.123 mol/3.123 mol = 1.000
H = 4.17 mol/3.123 mol = 1.34
O = 3.643 mol/3.123 mol = 1.167

1.167 = 7/6

Multiply the mole ratio for each element x 6.

C = 1.00 x 6 = 6
H = 1.33 x 6 = 7.98 = 8
O = 1.167 x 6 = 7.002 = 7

The empirical formula for citric acid is $\text{C"_6"H"_8"O"_7}$.