Empirical Formula
When scientists discover a new compound they need to experiment to determine the chemical
formula. They can do experiments where the percent composition of the new compound is
found or they can measure how many grams of each element is present directly. Based on this
data they can determines the lowest whole number ratio between particles using the chemistry
counting unit=the mole. It is important to remember that empirical formulas are experimental
formulas and will always be the lowest common ratio between the elements. CHO is an
empirical formula. C6H12O6 is NOT an empirical formula; all of the elements can be divided by
6!
Empirical Formula: The simplest ratio of the atoms present in a molecule.
Circle the 6 empirical formulas in the list below:
N2O2 CH4 H
2O2 C
6H12O6 MgSO4
NH3 H
2O NO CaCO3 C
12H22O11
The goal is for you to actually calculate an empirical formula when given either the %
composition or the mass of each element present.
Directions:
Step 1: If you are given percent composition pretend the percents are grams and go to the
next step. (Really- just change the % to a g for grams!!!)
Step 2: Convert the mass of each element to moles of each element using the atomic
masses from the periodic table.
Step 3: Find the ratio of the moles of each element by dividing the number of moles of
each by the smallest number of moles. (Your answers should be whole numbers and are
used as the subscripts in the chemical formula)
Step 4: Use the mole ratio to write the empirical formula
Example:
Find the empirical formula for the compound that contains 42.05 g of nitrogen and 95.95
g of oxygen.
Step 1: Skip –you were given grams! (If you were given % you would just change the % sign
to a g for grams=EASY!!)
Step 2: N = 42.05g N ( 1 mol N ) = 3.001 mol N
( 14.01 g N)
O = 95.95g O ( 1 mol O ) = 5.997 mol O
( 16.00 g O)
