The empirical formula of the oxide is Co₂O₃.
Step 1. Calculate the mass of oxygen
Your reaction is
Cobalt + oxygen ⟶ cobalt oxide
29.05 g + x g ⟶ 40.88 g
According to the Law of Conservation of Mass, the total mass of the reactants must equal the total mass of the products. Thus,
29.05 g + x g ⟶ 40.88 g
x = 40.88 – 29.05 = 11.83
Step 2. Calculate the moles of each element
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 Co to O.
Moles of Co = 29.05 g Co × (1 mol Co /(58.93 g Co) = 0.492 96 mol Co
Moles of O = 11.83 g O × (1 mol O/16.00 g O) = 0.739 38 mol O
Step 3. Calculate the molar ratio of the elements
Divide each number by the smaller number of moles
Co:O = 0.429 26:0.739 38 = 1:1.4999
Step 4. Multiply each number by a factor that makes the ratio close to whole numbers
Multiply by 2. Then
Co:O = 2:2.998 ≈ 2:3
Step 5: Write the empirical formula
EF = Co₂O₃
