Determining molecular formula?

Question

A molecule with a molecular weight of approximately 110 g/mole is analyzed. The results show that it contains 10.05% of carbon, 0.84% of Hydrogen, and 89.10% Chloride. Calculate the molecular formula of this compound.

I converted all of the percent comps. to moles of each and got .8375 mol C, .84 mol H, and 1.25 mol Cl2. I divided each by .8375, getting 1 mol C, 1 mol H, and 1.5 mol Cl2. Then I multiplied them all by 2 to have whole number and a empirical formula of C2H2(Cl2)3, but the empirical formula mass ends up being greater than the molecular mass and I cannot find a molecular formula. What am I doing wrong?

Answer 1

When you do problems of this type you must work in ATOMS, not MOLECULES. You cannot write a chemical formula as C2H2(Cl2)3

1 Divide each % by atomic mass:

C = 10.05/12.0= 0.8375

H = 0.84/1 = 0.84

Cl = 89.1/35.5 = 2.50

Divide all by 0.8375

C = 1.0

H=1.0

Cl = 2.976

Empirical formula =  CHCl 3 This is trichloromethane, (chloroform) which has a molar mass of 120. But you did give molar mass as approx 110.