# What is "sulfur trioxide"?

$S {O}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow S {O}_{3} \left(g\right)$
$S {O}_{3} \left(g\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{2} S {O}_{4} \left(a q\right)$
Oleum, fuming sulfuric acid is concentrated sulfuric acid saturated (sometimes it is less concentrated) with $S {O}_{3} \left(g\right)$.