Empirical and
Molecular Formulas
Chemical Formulas


tell type & number atoms in compound
microscopic level:
• imagine 1 atom, molecule, or formula unit
• formula indicates atom ratios

macroscopic level:
• imagine working in lab
• formula indicates mole ratios
Formulas
ex: 2H2O can mean
• 2 molecules of water
• 2 moles of water
2H2O molecules contain:
4 H atoms & 2 O atoms
2 moles of H2O molecules contain:
4 moles of H atoms & 2 moles of O atoms
What can say about
CuSO45H2O?
CuSO45H2O

this is a hydrated salt
• 1 mole CuSO4 contains 5 moles water
incorporated into its lattice structure

heat these salts:
• water is driven off leaving behind the
anhydrous salt (CuSO4)


mole ratio in formula can be used to predict
how much water found in any size sample
if had 2 moles of CuSO452O, how much
water would you lose on heating?
• ratio of salt to water is:

1mole salt to 5 moles water
Empirical Formulas
Empirical Formula

smallest whole # ratio of elements in compound

Ionic compounds:
• metal & non-metal elements
• only written as empirical formulas

Covalent compounds:
• only non-metal elements
• written as both empirical AND molecular formulas
• molecular formulas = multiples of empirical formulas
Molecular Formulas
Molecular Formulas


gives exact composition of molecule
molecular compounds have both
empirical and molecular formulas
• can be same or different
Relationship between empirical and
molecular formulas

molecular formula is whole number
multiple of empirical formula

molecular formula = n (empirical formula)

n is small whole number
• multiplies subscripts
• if n = 1 empirical & molecular formulas are the same
To Find Molecular Formula

If know empirical formula and molar
mass, can find molecular formula
• step 1: find mass of empirical formula
• step 2:__molar mass__ = whole number (n)
empirical mass
• step 3: multiply subscripts in
empirical formula by n

Find molecular formula for substance
whose empirical formula is CH and whose
molar mass is 78.0 g.
• step 1:
empirical mass = 1(12g) + 1(1g) = 13.0 g
• step 2:
molar mass = n
empirical mass
• Step 3: 6 x subscripts in CH
78.0g = 6
13.0g
C6H6