Empirical Formula
Empirical formula of a compound shows the simplest whole number ratio of the different types of atoms in a compound.
Steps to determine empirical formula of a compound
 Identify the various elements in the compound
 Find the mass of each element in the compound
 Calculate the number of moles of each element in the compound
 Divide each value of the number of moles by the smallest number. (E.g. if a compound has 0.02 moles of X and 0.06 moles of Y, we will have $\frac{0.02}{0.02} = 1$ and $\frac{0.06}{0.02} = 3$ after this step)
 Multiply each number by an integer so that the results are all whole numbers
 The whole numbers are then the subscripts of the elements in the formula
Examples:
A sample of iron sulphide contains 10.750 g of iron and 9.254 g of sulphur. What is the empirical formula of the compound?

Iron (Fe) Sulphur (S) Mass (g) 10.750 9.254 No. of moles $\frac{10.750}{56} = 0.192$ $\frac{9.254}{32} = 0.289$ Divide by smallest no. $\frac{0.192}{0.192} = 1$ $\frac{0.289}{0.192} = 1.51$ Multiply to get whole number $1 \times 2 = 2$ $1.51 \times 2 \approx 3$ Simplest Ratio 2 3  Empirical formula of the compound is $\text{Fe}_{2}\text{S}_{3}$
Consider a compound that contains $23.3 \%$ magnesium, $30.7 \%$ sulphur and $46.0 \%$ oxygen by mass. Find the empirical formula

Magnesium (Mg) Sulphur (S) Oxygen (O) Mass per 100g of compound 23.3 30.7 46.0 No. of moles $\frac{23.3}{24} = 0.97$ $\frac{30.7}{32} = 0.96$ $\frac{46}{16} = 2.88$ Divide by smallest no. $\frac{0.97}{0.96} \approx 1$ $\frac{0.96}{0.96} = 1$ $\frac{2.88}{0.96} = 3$ Simplest Ratio 1 1 3  Empirical formula of compound is $\text{MgSO}_{3}$
Molecular Formula
The molecular formula shows the actual number of the component elements in a compound. It may be same as or a multiple of the empirical formula. (Remember that empirical formula is the simplest)
 $\text{Molecular formula} = n \times \text{Empirical formula}$
 n is calculated from: $n = \frac{\text{Molecular mass}}{\text{Empirical formula mass}}$
 E.g. The empirical formula of $\text{C}_{6}\text{H}_{12}\text{O}_{6}$ is $\text{C}\text{H}_{2}\text{O}$
Steps to determine molecular formula of a compound
 Find the empirical formula of the compound
 Calculate the empirical formula mass
 Calculate the ratio n
 Multiply the subscripts in empirical formula by n
Example:
Consider a compound with an empirical formula of $\text{CHO}_{2}$ and a molecular mass of 90.1. Find the molecular formula of the compound.
 Empirical formula mass: $12 + 1 + 2(16) = 45$
 n: $n = \frac{90.1}{45} = 2$
 Molecular formula: $\text{C}_{2}\text{H}_{2}\text{O}_{4}$