The minimum number of moles of O₂ required for complete combustion of 1 mole of propane and 2 moles of butane is _____.

Solution

Chemistry ma’am · 3 steps

Balance each burning separately, scale by its moles, then add the oxygen together.

1. Burn propane. Complete combustion turns all carbon into CO₂ and all hydrogen into H₂O. For propane, C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, so 1 mole of propane needs 5 mol O₂.
2. Burn butane. For butane, C₄H₁₀ + 13/2 O₂ → 4CO₂ + 5H₂O. The right side has 8 + 5 = 13 oxygen atoms, which is 6.5 mol O₂ per mole of butane. For 2 moles, that is 2 × 6.5 = 13 mol O₂.

   n_(O₂) = 1 × 5 + 2 × 6.5 = 5 + 13 = 18

3. So. The minimum oxygen needed is 18 mol.

The common slip: 31. Taking 13 mol O₂ per mole of butane (from the doubled equation 2C₄H₁₀ + 13O₂) and then multiplying by 2 again gives 5 + 26 = 31, not 18.

The idea to keep. For any hydrocarbon CₓH_y, one mole burns completely with x + y/4 moles of O₂. Find this per mole first, then multiply by the moles you actually have.

Answer from the NTA final answer key. Solution by PrepWiser.