Number of oxygen atoms present in chemical formula of fuming sulphuric acid is ___________.

Solution

Chemistry ma’am · 4 steps

Fuming sulphuric acid is oleum, H₂S₂O₇. Build it from SO₃ and H₂SO₄ and count the O.

1. Name the compound. Fuming sulphuric acid is oleum. It forms in the contact process when SO₃ is absorbed in concentrated H₂SO₄.

   SO₃ + H₂SO₄ → H₂S₂O₇

2. Count from the equation. SO₃ brings 3 oxygen atoms and H₂SO₄ brings 4, so oleum has 3 + 4 = 7.
3. Check with structure. In HO-S(O₂)-O-S(O₂)-OH there are 2 OH oxygens, 4 S=O oxygens and 1 bridging S–O–S oxygen. That is 2 + 4 + 1 = 7.
4. So. The formula H₂S₂O₇ has 7 oxygen atoms.

The common slip: 4. Counting the oxygens of ordinary sulphuric acid, H₂SO₄, gives 4, not 7. Fuming acid carries an extra SO₃.

The idea to keep. Oleum is H₂SO₄ that has taken up one extra SO₃. If you remember that, you can always rebuild H₂S₂O₇ and count its atoms.

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