Question

Calculate the percentage of free SO₃ in an oleum (considered as a solution of SO₃ in H₂ SO₄ ) that is labelled '109% H₂SO₄ '.

Answer 1

109% H₂SO₄ ' refers to the total mass of pure H₂SO₄ , i.e., 109 g that will be formed when 100 g of oleum is diluted by 9 g of H₂O which (H₂O) combines with all the free SO₃ present in oleum to form H₂SO₄

H₂O + SO₃ → H₂SO₄

1 mole of H₂O combines with 1 mole of SO₃

or 18 g of H₂O combines with 80 g of SO₃

or 9 g of H₂O combines with 40 g of SO₃ .

Thus, 100 g of oleum contains 40 g of SO₃ or oleum contains 40% of free SO₃