1. Empirical and Molecular Formulas
Section 3.5
Empirical and Molecular Formulas

2. Empirical Formulas
Tells us relative number of atoms of each element it contains
Example: H2O: 2 atoms of H per 1 atom of O
ALSO: H2O: 2 mol of H per 1 mol of O
Mole concept allows us to calculate the empirical formula

3. Example Compound is 73.9% Hg and 26.1% Cl. Find empirical formula.
Assume 100 grams
73.9 g Hg x (1 mol Hg) = mol Hg
200.6 g Hg
26.1 g Cl x (1 mol Cl) = mol Cl
35.5 g Cl
mol Cl = mol Cl = mol Cl = 2 = HgCl2
mol Hg mol Hg = mol

4. Sample Exercise 3.13 p. 97
Ascorbic acid: 40.92% C, 4.58% H, and %O
Always divide the larger numbers (C and H) of moles by the smallest number of moles (O)

5. Molecular Formula Have to find Empirical Formula first!!!
Whole number multiple = molecular weight
empirical formula weight
Multiple each subscript of the empirical formula by the whole number multiple

6. Back to Sample Exercise 3.13
Empirical formula = C3H4O3
1st, find empirical formula weight:
3(12.0) + 4(1.0) + 3(16.0) = 88.0 amu
Given: experimentally determined molecular weight = 264 amu
W.N.M.= molecular weight = 264 = 3
empirical formula weight
Molecular Formula = C9H12O9

7. Homework
even only on page