Question

How can I calculate the empirical formula of an oxide?

Answer 1

You calculate the molar ratios of each element in the oxide.

EXAMPLE

When a 2.50 g sample of copper is heated, it forms 3.13 g of an oxide. What is its empirical formula.

Solution

Step 1. Determine the masses

Mass of Cu = 2.50 g

Mass of O = (3.13 – 2.50) g = 0.63 g

Step 2. Determine the moles

Moles of Cu = 2.50 g Cu × `(1"mol Cu")/(63.55"g Cu")` = 0.3934 mol Cu

Moles of O = 0.63 g O × `(1"mol O")/(16.00"g O")` = 0.394 mol Cu

Step 3. Determine the Molar Ratios

`"Moles of Cu"/"Moles of O" = (0.3934"mol")/(0.394"mol") = 1/1.00 ≈ 1/1`

Step 4. Write the Empirical Formula

The empirical formula of the oxide is CuO.