Empirical Formulas
Empirical formula tells the relative
number of atoms of each element in a
compound
Mole concept provides a way of
calculating the empirical formula
Also called the simplest formula
Example
Phosgene, a poison gas used during
World War I, contains 12.1 percent C,
16.2 percent O, and 71.7 percent Cl.
What is the empirical formula of
phosgene?
Step 1
Assume a 100g sample.
12.1 g C
16.2 g O
71.7 g Cl
Step 2
Change each mass into moles
1.01 mol C
1.01 mol O
2.02 mol Cl
Step 3
Divide each mole through by the
smallest to obtain a ratio
1 C: 1O: 2Cl
Step 4
Write the formula
COCl2
Example 2
A 5.325 g sample of methyl benzoate, a
compound used in perfumes, is found
to contain 3.758 g of carbon, .316 g of
hydrogen, and 1.251 g of oxygen. What
is the empirical formula?
C4H4O
Molecular Formula
The empirical formula may or may not
be the true molecular formula
To obtain the molecular formula, we
must know the molecular weight
The molecular weight of methyl
benzoate is 136.0 amu. Is the empirical
formula we determined the molecular
formula?
What is the molecular formula?
Combustion Example
The combustion of 1.000 g of ascorbic
acid produces 1.50 g of CO2 and .405 g
of H2O. Determine the empirical
formula.
Find grams of each substance
Find moles of each
Divide to find ratio