Contact Process and Sulfuric Acid H₂SO₄

Sulfuric acid H₂SO₄ (98.07 g/mol) is manufactured at enormous scale via the contact process. Elemental sulfur burns to sulfur dioxide SO₂ (64.07 g/mol); catalytic oxidation gives sulfur trioxide SO₃ (80.07 g/mol); absorption in concentrated acid yields oleum and then H₂SO₄. Each step's mass balance requires correct molar masses — confusing SO₂ with SO₃ shifts yield predictions by 25%.

The equilibrium 2 SO₂ + O₂ ⇌ 2 SO₃ favors product at low temperature and high pressure, but the catalyst needs heat. Industrial plants optimize conversion rather than assume 100% yield — the same percent-yield framework used when students synthesize aspirin C₉H₈O₄ from salicylic acid. Unreacted SO₂ is recycled; environmental regulations cap emissions.

Dilution and concentration specs use molarity and mass percent. A 98% H₂SO₄ solution is not 98 M — density matters. Laboratory dilution always adds acid to water because the hydration exotherm is dangerous. Compare phosphoric acid H₃PO₄ (97.99 g/mol) and hydrochloric acid HCl (36.46 g/mol): same molarity preparation logic, different hazards and molar masses.