10.3. Chemical Formulas http://www.ck12.org
FIGURE 10.6
Acetic acid (left) has a molecular formula
of C 2 H 4 O 2 , while glucose (right) has a
molecular formula of C 6 H 12 O 6. Both have
the empirical formula CH 2 O.Empirical formulas can be determined from the percent composition of a compound. In order to determine its
molecular formula, it is necessary to also know the molar mass of the compound. Chemists have various methods
to determine the molar mass of an unknown compound. In order to go from the empirical formula to the molecular
formula, follow these steps:
- Calculate the empirical formula mass (EFM), which is simply the molar mass represented by the empirical
formula. - Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number
or very close to a whole number. - Multiply all of the subscripts in the empirical formula by the whole number found in step 2. The result is the
molecular formula.
Example 10.17
The empirical formula of a compound that contains boron and hydrogen is BH 3. Its molar mass is 27.7 g/mol.
Determine the molecular formula of the compound.
Answer:
Follow the steps outlined above.
- The empirical formula mass (EFM) = 13.84 g/mol
molar mass
EFM
=
27. 7
13. 84
= 2
3. BH 3 ×2 = B 2 H 6
The molecular formula of the compound is B 2 H 6. The molar mass of the molecular formula matches the molar mass
of the compound.
You can watch a video lecture about molecular and empirical formulas at http://www.khanacademy.org/science/c
hemistry/chemical-reactions-stoichiometry/v/molecular-and-empirical-formulas.
You can watch a video lecture about determining molecular and empirical formulas from percent composition