The Mole & Chemical FormulasThe Mole & Chemical Formulas

A chemical formula represents the ratio of atoms that always exists for that compound

Example: Water – H2O Always 2 H atoms to 1 O atom

The Mole & Chemical FormulasThe Mole & Chemical Formulas

This also means if you had a mole of water…

You would have 2 moles of Hydrogen atoms

And 1 mole of Oxygen atoms

Empirical FormulaEmpirical Formula

This is the simplest ratio of atoms in a chemical formula.

Example: Glucose, a sugar, has the molecular

formula C6H12O6

The empirical formula is CH2O

Empirical FormulaEmpirical Formula

Example: A chemical has 80.0% C atoms and

20.0% H atoms. What is the empirical formula of the compound?

Use the %’s as masses to find moles…

Empirical FormulaEmpirical Formula

80.0 g C 1 mol C12.0 g C

= 6.67 mol C

20.0 g H 1 mol H1.0 g H

= 20.0 mol H

Empirical FormulaEmpirical Formula

Use the moles to determine the ratio:

Carbon:6.67 mol

6.67 mol= 1

Hydrogen:20.0 mol

6.67 mol= 3

CH3

This is the Empirical Formula!

Molecular FormulaMolecular Formula

This gives the actual number of atoms for each element in a compound.

Example: If the molar mass of the “CH3” compound

from the previous problem is 30.0 g/mol, what is the molecular formula?

Molecular FormulaMolecular Formula

Empirical formula mass for CH3 is 15.0 g/mol

Then find the ratio of molar to empirical mass.

30.0 g/mol (molecular)

15.0 g/mol (empirical)= 2.00

Molecular FormulaMolecular Formula

Multiply the empirical formula by your new ratio:

C(1x2)H(3x2) = C2H6 (molecular formula)

Molecular FormulaMolecular Formula

Example: A chemical is 48.4% C atoms, 8.12%

H atoms and the rest is Oxygen. If the molecular mass is 222 g/mol, find the molecular formula.

First find empirical formula…

Molecular FormulaMolecular Formula

48.4 g C 1 mol C12.0 g C

= 4.03 mol C

8.12 g H 1 mol H1.0 g H

= 8.12 mol H

43.5 g O 1 mol O16.0 g O

= 2.72 mol O

Molecular FormulaMolecular Formula

Carbon:4.03 mol

2.72 mol= 1.48

Hydrogen:8.12 mol

2.72 mol= 2.99

x 2 = 3 C3

Oxygen:2.72 mol

2.72 mol= 1.00

x 2 = 6

x 2 = 2

H6O2

Molecular FormulaMolecular Formula

222 g/mol (molecular)

74.0 g/mol (empirical)= 3.00

C3x3 H6x3 O2x3 C9H18O6

Molecular Formula!