# Can the empirical formula of a compound be experimentally found?

Jun 23, 2014

The empirical formula of a compound can be experimentally found.

For example, you can do an experiment to determine the empirical formula of magnesium oxide. You heat a known mass of magnesium in a crucible and determine the mass of oxide formed.

Example

Assume that you heated 0.297 g of magnesium and obtained 0.493 g of the oxide. What is the empirical formula of magnesium oxide?

Solution

The empirical formula is the simplest whole-number ratio of atoms in a compound.

The ratio of atoms is the same as the ratio of moles. So our job is to calculate the molar ratio of Mg to O.

Mass of Mg = 0.297 g

Mass of oxide = mass of Mg + mass of O

0.493 g = 0.297 g + mass of O

Mass of O = (0.493 – 0.297) g = 0.196 g

Moles of Mg = 0.297 g × $\left(1 \text{mol Mg")/(24.30"g Mg}\right)$ = 0.012 22 mol Mg

Moles of O = 0.196 g O × $\left(1 \text{mol O")/(16.00"g O}\right)$ = 0.012 25 mol O

Ratio = "Moles of Mg"/"Moles of O" = (0.012 22"mol")/(0.012 25"mol") = 1/1.002 ≈ 1/1.

There is 1 mol of Mg for 1 mol of O. So

There is 1 atom of Mg for 1 mol of O.

The empirical formula of magnesium oxide is MgO.