Oleum

Oleum (Latin oleum = "oil"), or fuming sulfuric acid refers to a solution of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid).

Oleums can be described by the formula ySO₃.H₂O where y is the total molar sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H₂SO₄.xSO₃ where x is now defined as the molar free sulfur trioxide content. Oleum is generally assayed according to the free SO₃ content by weight. It can also be expressed as a percentage of sulfuric acid strength; for oleum concentrations, that would be over 100%. For example, 10% Oleum can also be expressed as H₂SO₄.0.10889SO₃, 1.0225SO₃.H₂O or 102.25% Sulfuric Acid. The conversion between % acid and % oleum is: % Acid = 100 + 18/80 * % Oleum

A value for x of 1 gives us the empirical formula H₂S₂O₇ for disulfuric (pyrosulfuric) acid. Pure disulfuric acid itself is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes.