Empirical Formula

The empirical formula of a compound is the simplest molar ratio of its elements. We can use the empirical formula to then determine the molecular formula of an unknown compound through a series of reactions and calculations.

Calculating Empirical Formula

1. Calculate the mass (or, alternatively, percentage by mass) of each element based on the information given (only convert to percentages if the information is given from two different samples)

2. Convert the mass of each element to moles

3. Find the simplest molar ratio by dividing each molar amount by the smallest value

4. If the ratios are not close enough to round, multiply the values by the lowest common factor which results in all the values being in a range of +/- 0.05 from a whole number

5. Write out the empirical formula of the compound based on the ratios of the elements

Calculating Molecular Formula

From an empirical formula, we can use one final piece of data to work out the molecular formula of the unknown compound - molar mass.

1. Using the molar mass of the unknown compound, we can find the ratio between the molecular formula and the empirical formula

2. Once we have determined the ratio, we then multiply the empirical formula by this ratio to arrive at the molecular formula of this compound

Empirical Formula Worked Example | Elucidate Video

In the above video, Henry works through and explains an empirical formula question from a previous WACE exam.