Determining Empirical
Formula
Empirical Formulas

Indicate the lowest whole number ratio of
atoms of each element in a compound.

The molecular formula of hydrogen peroxide
is H2O2 the empirical formula would be…?

HO

Ho, Ho, Ho…
The molecular formula of glucose is…
 C6H12O6, , its empirical formula is…
 CH2O …
 So what’s my point?


Empirical formulas are lowest whole
number ratios of atoms of each type …to
determine them experimentally we need
numbers of atoms!
Consider the following:
Analysis of a compound at CSI North
Haven Labs indicates that it contains
 32.38 g Na
 22.65 g S
 44.99 g O
 How can we use this data to determine the
formula?...Think…
 Mass  number of atoms!

Remember doing mole
conversions?
Use the molar mass of element convert
to moles.
 A mole is a number of atoms … you now
have a ratio of atoms in the compound.
 Divide each mole amount by the smallest
mole amount… whole # ratio???
 Use ratio to write subscripts


1.408 mol Na : 0.7063 mol S : 2.812mol O

Divide each by smallest 

1.993 mol Na : 1 mol S : 3.981 mol O
 Na2SO4
Try this..
Analysis indicates that a compound
contains 78.1% Boron and 21.9%
hydrogen determine the empirical formula.
 Why is this one any different from the last
example?
 Assume you have a 100 gram
sample…now how many grams of boron
do you have?

7.22 mol B : 21.7 mol H
 Divide through by smallest 
 1 mol B
: 3.01 mol H


BH3
Try this one …

Analysis of a 10.150 g sample of a
compound known to contain only
phosphorus and oxygen indicates a
phosphorus content of 4.433 g. What is
the empirical formula?

Grams of P = 4.433g Grams of O = ?

Now try it…

0.1431 mol P : 0.3573 mol O

Divide by smallest 

1 mol P : 2.497 mol O …

Now what?...Multiply through by two to
obtain whole numbers… 2 : 5
P2O5
Find empirical formulas…
1.
36.48% Na, 25.41% S, 38.11 % O…
2.
53.70 % iron and 46.30% sulfur…
3.
4.
1.04 g K, 0.70 g Cr and 0.86 g O…
37.51% C, 4.20% H and 58.29 % O …
Quantitative Analysis

Various methods are used to determine
the mass or percent composition of each
element in a given compound…more
about those later…