Empirical and Molecular Formulas

The content that follows is the substance of lecture 9. In this lecture we cover the relationship between Empirical and Molecular formulas and the calculations used to determine one from the other.

The Empirical Formula

An Empirical formula is the chemical formula of a compound that gives the proportions (ratios) of the elements present in the compound but not the actual numbers or arrangement of atoms.

This would be the lowest whole number ratio of the elements in the compound.

For Example:

Image result for empirical formula



In order to determine the Empirical formula for a compound or molecule, we need to know the mass percentages of the the elements in the compound. Once we have this information we can convert it to moles to determine the ratios between the elements.

A simple rhyme can be used to remember the process:

Percent to Mass

Mass to Mole

Divide by Small

Multiply 'til Whole


For Example:

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.

bullet If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.

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.

bullet 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 mass of the empirical formula and setting up a ratio:



Some more detailed instructions:



Here are some practice problems with answers: