• The molecule has 2 hydrogen atoms and 2 oxygen atoms. Let’s
start with hydrogen.
• The atomic weight of hydrogen is 1 amu. Since there are 2
atoms, we have a total of 2 × 1, or 2 amu.
• Now let’s talk about the oxygen in the molecule. The atomic
weight of oxygen is 16, and since there are 2 atoms, the total is 16
× 2, or 32 amu.
• The total is 2 + 32 = 34. The formula weight for H 2 O 2 is 34
amu.Here’s another example. Let’s figure out the weight for sulfuric acid: H 2 SO 4.
• The molecule has 2 hydrogen atoms, 1 atom of sulfur, and 4
oxygen atoms. Let’s start with hydrogen.
• The atomic weight of hydrogen is 1 amu. We have 2
hydrogens, so there are a total of 2 amu.
• The atomic weight of sulfur is 32 amu. So we have 32 amu of
sulfur.
• Now let’s talk about the oxygen. We know the atomic weight
of oxygen is 16 and there are 4 atoms. So we have a total of 16 ×
4, or 64 amu.
• 2 + 32 + 64 = 98. The formula weight for H 2 SO 4 is 98 amu.
Empirical Formula: An empirical formula shows the ratio of atoms
within a molecule. To find an empirical formula from a molecular formula,
first find the largest whole number by which all of the subscripts in the
molecular formula are divisible. Then divide each subscript by that number.
Let’s look at a few examples.
• We talked about the molecular formula of hydrogen peroxide,
which is H 2 O 2 . The largest number that can divide into 2 is 2. If
we factor a 2 out of both subscripts, we’re left with HO. So the
empirical formula for hydrogen peroxide is HO.
• Ethane has the molecular formula C 2 H 6 . Let’s find the
empirical formula. What’s the largest number that goes evenly
into 2 and 6? It’s 2. If we divide both subscripts by 2, we get an
empirical formula of CH 3.