Chapter 7
Chemistry
Empirical Formulas/molecular
formulas
LOOK AT: Pages
245-249
1
Empirical vs Molecular Formula
Molecular formula
true or actual ratio
of the atoms in a
compound.
Empirical formula Smallest whole
number ratio of
atoms in a
compound
C2H6
CH3
Types of Formulas
The formulas for compounds and molecules
can be expressed as an empirical formula
and as a molecular formula.
Empirical
Molecular
CH
C 2H 2
CH
C 6H 6
CO2
CO2
CH2O
C5H10O5
3
Calculating Empirical Formula
1. Given: mass or percent of each element. If
percent, just change % to g.
2. Convert grams to moles.
3. Divide ALL mole ANSWERS by the smallest number
moles . By doing this, you find the subscripts. You
may have to round to a whole number….the number
can only be off by a little.
4. If any of your subscripts turn out to be .25,
.50, or .75, you will need to multiply by 2, 3,
or 4 to get a whole number.
Practice
• Find the empirical formula of a
compound that contains 75.0 %
Carbon and 25.0% Hydrogen.
• Find the empirical formula for a
sample of 25.9% N and 74.1% O.
5
practice
• A typical charcoal briquette that is used in
making a BBQ fire is composed of 43.2 g of
Carbon. When the Charcoal lump is burned, it
combines with oxygen and the resulting
compound has a mass of 159.0 g. What is the
empirical formula of the compound?
6
Empirical formulas of hydrates
EX: A 5.00 g sample of a hydrate Copper (II)
Nitrate is heated and 3.90 g of the
anhydrous salt remains. Calculate the
empirical formula of the hydrate. Name the
hydrate.
7
Finding molecular formula from the empirical
formula
1. Find the molar mass of the empirical formula
2. Divide the molar mass of the empirical
formula by the molar mass of the molecular
formula(will be given in the problem)--- you
should get a whole number.
3. Take this number and multiply it to each
subscript in the Empirical Formula.
4. The new formula is the molecular formula.
8
Molecular Formula
• The empirical formula for ethylene is
CH2. Find the molecular formula if
the molecular mass is
28.1 g/mol?
Practice
A compound has a molar mass of 176.0
and an empirical formula of C3H4O3. What
is the molecular formula?
10