I II III

Formula’s

The Mole

Formulas

molecular formula = (empirical formula)n molecular formula = C6H6

empirical formula = CH

Empirical formula: the lowest whole number ratio of atoms in a compound.

Molecular formula: the true number of atoms of each element in the formula of a compound.

Formulas (continued)

Formulas for ionic compounds are Formulas for ionic compounds are ALWAYSALWAYS empirical (lowest whole number empirical (lowest whole number ratio).ratio).

Examples:Examples:

NaCl

MgCl2

Al2(SO4)3

K2CO3

Formulas (continued)

Formulas for molecular compounds Formulas for molecular compounds MIGHTMIGHT be empirical (lowest whole be empirical (lowest whole number ratio).number ratio).

Molecular:Molecular:

H2O

C6H12O6 C12H22O11

EmpiricalEmpirical::

H2O

CH2O C12H22O11

Empirical Formula

Empirical comes from Latin empiricus meaning a doctor relying on experience. An empirical formula must be obtained from experimental data.

The percent composition of a compound is the data you need to calculate the basic ratio of the elements contained in the compound.

The basic ratio, is called the empirical formula, gives the lowest whole-number ratio of the atoms of the elements in a compound.

Empirical Formula

C2H6

CH3

reduce subscripts

Smallest whole number ratio of atoms in a compound

Empirical Formula…how to find it!

1. Find mass (or %) of each element.

2. Find moles of each element.

3. Divide moles by the smallest # to find subscripts.

4. When necessary, multiply subscripts by 2, 3, or 4 to get whole numbers’s.

Empirical Formula

Find the empirical formula for a sample of 25.9% N and 74.1% O.

25.9 g 1 mol

14.01 g = 1.85 mol N

74.1 g 1 mol

16.00 g = 4.63 mol O

1.85 mol

1.85 mol

= 1 N

= 2.5 O

Empirical Formula

N1O2.5Need to make the subscripts whole

numbers multiply by 2

N2O5

Molecular Formula “True Formula” - the actual number

of atoms in a compound

CH3

C2H6

empiricalformula

molecularformula

?

Molecular Formula1. Find the empirical formula.2. Find the empirical formula mass.3. Divide the molecular mass by the empirical mass.4. Multiply each subscript by the answer from step 3.

nmass EF

mass MF nEF

Molecular Formula The empirical formula for ethylene is

CH2. Find the molecular formula if the molecular mass is 28.1 g/mol?

28.1 g/mol

14.03 g/mol = 2.00

empirical mass = 14.03 g/mol

(CH2)2 C2H4