Molecular formula is the formula that shows the actual number of atoms of each type of element present in a compound.

Empirical Formula is the formula that shows the number of atoms of each type of element present in the compound in the lowest possible ratio. It is the simplest formula of the compound.

Determining an empirical formula of a compound

To determine the empirical formula of a compound from its composition expressed in percentage by mass of each element, we use the following steps.

1. Write the symbols of the constituent elements.
2. Write the percentage by mass of each element below its chemical symbol.
3. Divide the percentage by mass of the element by the respective atomic mass to get the number of moles of each element.
4. Divide the numbers of moles of all element by the smallest number of mole obtained in step 3 to get their mole ratio.
5. Write the symbols of the constituent elements side by side and the numbers obtained as subscripts to get its empirical formula.

Determining molecular formula of a compound

As explained above, some compounds have the same empirical formula and molecular formula. In case of these compounds, the molecular formula is a whole number multiple of the empirical formula. In addition to that, molecular mass of the compound is also a multiple of the empirical formula mass. That is:

Molecular formula = Empirical formula × n
Molecular mass = Empirical formula mass × n
Where ‘n’ is a number greater or equal to 1.

It is possible to determine the molecular formula of a compound from its percentage composition and the molecular mass of the compound using the following steps.

1. Determine the empirical formula of the compound.
2. Calculate the empirical formula mass.
3. Divide the molecular mass by the empirical formula mass to get the number that should multiply the empirical formula.
4. Multiply the empirical formula by the number obtained when the molecular mass is divided by the empirical formula mass.