Empirical formulas don't distinguish between molecules with different molecular formulas that happen
to have the same percent composition. Check out these molecules, which all have the empirical
formula of CH:
C2H2
C3H3
C4H4
C5H5
C6H6
26.04 g/mol
39.06 g/mol
52.08 g/mol
65.10 g/mol
78.11 g/mol
You can distinguish these molecules by their molecular mass, though. If you know the molecular mass
of your compound (which you can determine with an instrument called a mass spectrometer), then you
can figure out which of these compounds you have!
1. Determine how much one “unit” of your empirical formula weighs
2. How many of those formula units fit into the given molecular mass?
3. Multiply the subscripts in the empirical formula by the number you found in Step 2.
Example
The empirical formula of ribose (a sugar) is CH2O. In a separate experiment, using a mass
spectrometer, the molar mass of ribose was determined to be 150 g/mol. What is the molecular formula
of ribose?
1. The “molar mass” of the empirical formula CH2O is
12.01+2(1.01 )+16.00=30.03 g/mol
2. The molar mass of ribose is 150 g/mol.
There are
150
30 =5
“empirical units” in each ribose molecule.
3. Multiply each of the subscripts in the empirical formula (CH2O) by 5: C5H10O5
Therefore, the molecular formula of ribose is C5H10O5.
Molecular Formula from Empirical Formula ChemistNATE
