Use percent composition information to calculate either the empirical formula or the molecular formula for a compound.

Problem: Trichloroethylene (TCE) is a common solvent used to degrease machine parts. Calculate the empirical formula for TCE if the percent composition is 18.25% C, 0.77% H and 80.99% Cl.

Solution: Consider having a 100 g sample of trichloroethylene. The percent composition is then 18.25 g C, 0.77 g H and 80.99 g Cl. Find the number of moles of each element.

18.25 g C (1 mol C/12.01 g C) = 1.520 mol C
0.77 g H (1 mol H/1.008 g H) = 0.76 mol H
80.99 g Cl (1 mol Cl/35.45 g Cl) = 2.285 mol Cl

Chemical formulas are written with whole numbers, so divide by the smallest number of moles to find the empirical formula.

Because C: 1.519/0.76 ≈ 2 and H: 0.76/0.76 = 1 and Cl: 2.285/0.76 ≈ 3, the empirical formula is C2HCl3.