Question

At elevated temperatures copper (Cu) reacts with oxygen (O2) to form copper oxide (CuO). If you have 120 grams of copper reacting with 1.00 moles of oxygen, which reagent will run out first?

Answer 1

The balanced equation for the reaction between Cu and O2 is

2 Cu + O₂ ---------> 2 CuO

Mass of Copper = 120 g

Molecular weight of Copper = 63.55 g/mol

Number of moles of Cu = mass/mol.wt. = 120 g/(63.55 g/mol) = 1.89 mol

According to balanced equation,

2 moles of Cu reacts with 1 mole of O₂

So, 1.89 mol of Cu would react with 1.89 mol * ½ = 0.944 mol of O₂

But the number of moles of O₂ (1.0 mol) present in excess than required.

Hence, O₂ is the excess reagent. Cu is the Limiting reagent.

Therefore, Cu runs out first.