To determine the number of molecules of sulfur trioxide (SO₃) produced in the reaction where 2 molecules of sulfur dioxide (SO₂) react with 1 molecule of oxygen (O₂), we can start by writing the balanced chemical equation for the reaction:
\[ 2 , \text{SO}_2 + \text{O}_2 \rightarrow 2 , \text{SO}_3 \]
From this equation, we can see that for every 2 molecules of sulfur dioxide and 1 molecule of oxygen consumed, 2 molecules of sulfur trioxide are produced.
Thus, in this reaction, 2 molecules of sulfur trioxide (SO₃) are produced.