 A chemical formula in which the ratio of the elements are in the lowest terms is called an empirical formula .

 The empirical formula for a glucose molecule (C
6
H
12
O
6
) is CH
2
O. All the subscripts are divisible by six.
C
6
H
12
O
6 6 6
6
C H
2
O

Some formulas, such as the one for carbon dioxide, CO
2
, are already empirical formulas without being reduced .

 To find the empirical formula from the molecular formula, you must divide all the subscripts by the (GCF) greatest common factor .

Determine the empirical formulas for each of the following molecular formulas.
1. C
8
4. C
3
H
18
2. H
2
O
2
3. Hg
2
H
…..______________
……______________
Cl
2
6
…..______________
…….______________
5. Na
2
C
2
O
4
..._____________
6. H
2
O......________________
7. C
4
H
8
….._______________
8. C
9. C
4
7
H
H
10. CH
6
…..________________
12
…._______________
3
COOH….___________

Finding empirical formulas using molecular mass:
 Find mass (or %) of each element.
 Find moles of each element.
 Divide moles by the smallest # to find subscripts.
 When necessary, multiply subscripts by
2, 3, or 4 to get whole #’s.

EXAMPLE: Find the empirical formula for a sample of 25.9% N and 74.1% O.

25.9g = 1.85 mol N 1.85mol N = 1 N x 2 = N
2
14g/mol 1.85 mol
 74.1 g = 4.63 mol O 4.63 mol O = 2.5 O x 2 = O
5
16 g/mol 1.85 mol
N
2
O
5

1.
A substance is 36.1% by weight calcium and 63.9% chlorine. What is the empirical formula of this compound?

2. A compound is 43.4%Na, 11.3%C, and
45.3% O. What is the empirical formula for this compound?

3. A compound is 2.46%H, 39.1%S, and
58.5%O. What is the empirical formula for this compound?

 A molecular formula is the “ true formula ” of a compound. The chemical formula for a molecular compound shows the actual number of atoms present in a molecule .

 Find the empirical formula.
 Determine the empirical formula mass.
 Divide the molecular mass by the empirical formula mass to determine the multiple.
 Multiply the empirical formula by the multiple to find the molecular formula.
MF mass = n
EF mass
(EF)n = molecular formula

The empirical formula for ethylene is CH
2.
molecular formula if the molecular mass is
Find the
28.1g/mol.
C = 1 x 12 = 12
H = 2 x 1 = +2
14g/mol = empirical formula mass
28.1 g/mol = 2
14 g/mol
(CH
2
)
2
 C
2
H
4

1. Find the molecular formula for a compound with a mass of 78 amu and the empirical formula CH.

2. Find the molecular formula for a compound with a mass of 82 amu and the empirical formula C
3
H
5
.

3. Find the molecular formula for a compound with a mass of 90 amu and the empirical formula HCO
2
.

4. Find the molecular formula for a compound with a mass of 112 amu and the empirical formula CH
2
.

5. Find the molecular formula for a compound with a mass of 40 amu and the empirical formula C
3
H
4
.

Writing Empirical Formulas:
1.Determine the empirical formula of a compound containing 2.644 g of gold and
0.476 of chlorine.

2. Determine the empirical formula of a compound containing 0.928 g of gallium and 0.412 of phosphorus.

9.
Find the molecular formula of a compound that contains 42.56g of palladium and 0.80g of hydrogen. The molar mass of the compound is 216. g/mol.