Empirical Formula
0 Consists of the symbols for the elements combined in
a compound, with subscripts showing the smallest
whole-number mole ratio of the different atoms in the
compound
EF from percentages – Step 1
0 Begin by converting % comp. to a mass composition.
0 Assume you have a 100 g sample.
0 Change % signs to grams:
0 Ex: The % composition of diborane is 78.1% B and
21.9% H.
0 Therefore, 100.0g of diborane contains 78.1g of Boron and
21.9 g of Hydrogen.
EF from Percentages – Step 2
0 Convert the mass composition of each element to a
molar composition.
0 Ex: 78.1 g B x
1 mol B
= 7.22 mol B
10.812 g B
21.9 g H x
1 mol H
1.008 g H
= 21.7 mol H
We now have a mole ratio of 7.22 moles of Boron to 21.7
moles of Hydrogen
However, this is NOT a ratio of SMALLEST WHOLE #s
EF from Percentages – Step 3
0 To find a ratio of smallest whole numbers, divide
each # of moles by the smallest number in the
existing ratio.
0 Ex:
7.22 mol B 21.7 mol H
:
7.22
7.22
= 1 mol B : 3.01 mol H
EF from Percentages - Answer
0 Because of rounding or experimental error , a
compound’s mole ratio sometimes consists of #s
close to whole numbers instead of exact whole
numbers.
0 Differences may be ignored and nearest whole
number taken.
0 Diborane contains atoms in the ration 1B : 3H.
0 Diborane’s EF is BH3
EF – Example 2
0 Quantitative analysis shows that a compound contains
32.38% sodium, 22.65% sulfur, and 44.99% oxygen.
Find the empirical formula of this compound.
EF – Example 3
0 Find the empirical formula of a compound found to
contain 26.56% potassium, 35.41% chromium, and
the remainder oxygen. Once the EF is found, name
the compound.
EF from mass – Step 1
0 Sometimes mass composition is known/given instead
of percent composition.
0 Convert mass composition to mole composition.
0 Ex: Calcium Bromide contains 4.00 g of Calcium and
16.00 g of Bromine.
0 4.00 g Ca x
1 mol Ca
40.078 g Ca
0 16.00 g Br x
= 0.0998 mol Ca
1 mol Br
79.904 g Br
= 0.2002 mol Br
EF from mass – Step 2
0 Calculate the smallest whole – number mole ratio to
atoms.
0 Ex:
0.0998 mol Ca 0.2002 mol Br
:
0.0998
0.0998
= 1 mol Ca : 2.006 mol Br
0 Calcium Bromide’s EF is CaBr2
EF – Example 4
0 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 of this compound?
EF – Example 5
0 Analysis of a compound indicates that it contains 1.04
g K, 0.70 g Cr, and 0.86 g O. Find its empirical formula.
Empirical Formula Review
0 Percent Comp. Given
0 % comp  mass
comp  comp in
moles  smallest
whole-number mole
ratio in atoms
0 Mass Comp. given
0 Mass comp  comp
in moles  smallest
whole-number mole
ratio in atoms
If a .5 value occurs in the last
step, multiply everything by 2
Molecular Formulas - Steps
0 1. Determine the EF
0 2. Determine the molar mass of the compound from
the EF
0 3. Divide the molecular mass of the compound by the
molar mass of the empirical formula
0 4. The whole number determined in step 3 is used to
multiply the subscripts in the empirical formula to
derive the subscripts in the molecular formula
Example:
0 Analysis of a compound containing chlorine and lead
reveals that the compound is 59.37% lead. The
molecular mass of the compound is 349.0 g/mol.
What is the molecular formula for this compound?
Example 2
0 Glycerol is a thick, sweet liquid obtained as a
byproduct of the manufacture of soap. Its percent
composition is 39.12% carbon, 8.75% hydrogen, and
52.12 % oxygen. The molecular mass is 92.11 g/mol.
What is the molecular formula for glycerol?
Example 3
0 You have isolated a compound and sent it for analysis
to help determine its identity. The lab sends back the
results indicating that the compound is 50.00%
carbon, 8.33% hydrogen, 19.44% nitrogen, and
22.22% oxygen. The molecular mass of the
compound is found to be 144.0 g/mol. What is the
molecular formula of the compound you isolated?
Example 4
0 A component of protein called serine has an
approximate molecular mass of 100.00 g/mol. If the
percent composition is as follows, what is the
molecular formula for serine?
0 C= 34.95%
0 H = 6.844%
0 O = 46.56%
0 N = 13.59%