Determination of Molecular Formulas
The formula calculated from percent composition by mass is always the empirical
formula because the subscripts in the formula are always reduced to the smallest
whole numbers. To calculate the actual, molecular formula we must know the approximate
molar mass of the compound in addition to its empirical formula. Knowing that
the molar mass of a compound must be an integral multiple of the molar mass of its
empirical formula, we can use the molar mass to fi nd the molecular formula, as
Example 3.11 demonstrates.