Molecular Formula - the actual number of atoms of each element present in a substance.
Examples sugar….
molecular formula - C6H12O6
empirical formula - C1H2O1
The empirical formula is just the reduced molecular formula.
water….
molecular formula - H2O
empirical formula - H2O
When the molecular formula cannot be reduced, it is the same as the empirical formula.
Once the empirical formula is found, the molecular formula for a compound can be determined if the
molar mass of the compound is known.
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Steps for Determining an Molecular Formula
Calculate the empirical formula.
Calculate the molar mass of the empirical formula.
Divide the molar mass of the molecular formula (should be given in the problem) by the molar
mass of the empirical formula. This will give you the ratio between the molecular formula and
the empirical formula.
Multiply all the atoms (subscripts) by this ratio to find the molecular formula.
Example Problem #1
NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O. Calculate the empirical formula of
NutraSweet and find the molecular formula. (The molar mass of NutraSweet is 294.30 g/mol)
Start with the number of grams of each element, given in the problem. Convert the mass of each
element to moles using the molar mass from the periodic table.
Divide each mole value by the smallest number of moles calculated. Round to the nearest whole
number.
This is the mole ratio of the elements and is represented by subscripts in the empirical formula.
 If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same factor to
get the lowest whole number multiple.
Now, we can find the molecular formula by finding the molar mass of the empirical formula and
setting up a ratio:
Molecular Formula =
For additional help, watch the video at https://www.youtube.com/watch?v=gZIL4rd0tkw
Practice Problems
Show ALL work on a separate sheet of paper for each of the following problems.
1. 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. If the molar mass is
184.22 g/mol, what is the molecular formula?
2. Determine the molecular formula for ibuprofen if the percent composition is 75.5% C, 8.80% H
and 15.5% O and the molar mass is 206 g/mol.
3. The hydrocarbon used in the manufacture of foam plastics is called styrene. Styrene contains
92.25% carbon and 7.75% hydrogen. What is the empirical formula of styrene? If the molar
mass of styrene is 104 g/mol, what is the molecular formula?
4. Analysis of eucalyptol (an oil found in the leaves of the eucalyptus tree) indicates it has a molar
mass of 462 g/mol and contains 77.87% C, 11.76% H, and 10.37% O. Determine the molecular
formula of eucalyptol.