Here's how to determine the simplest formula (empirical formula) of the copper oxide:

1. Find the mass of oxygen:

* Mass of oxygen = Mass of oxide - Mass of copper

* Mass of oxygen = 2.81 g - 2.50 g = 0.31 g

2. Convert masses to moles:

* Moles of copper = Mass of copper / Molar mass of copper

* Moles of copper = 2.50 g / 63.55 g/mol ≈ 0.0393 mol

* Moles of oxygen = Mass of oxygen / Molar mass of oxygen

* Moles of oxygen = 0.31 g / 16.00 g/mol ≈ 0.0194 mol

3. Find the mole ratio:

* Divide the number of moles of each element by the smallest number of moles:

* Copper: 0.0393 mol / 0.0194 mol ≈ 2

* Oxygen: 0.0194 mol / 0.0194 mol ≈ 1

4. Write the empirical formula:

* The mole ratio indicates that the simplest formula is Cu₂O.

Therefore, the simplest formula of the copper oxide is Cu₂O (copper(I) oxide).