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Problem 1 - Entrance Test

A mixture of 2 moles of SO₂ and 3 moles of O₂ is passed through a catalyst in a sealed container to form SO₃. If the reaction is 2SO₂ + O₂ → 2SO₃ with 75% efficiency, what is the total number of moles of gas in the container at equilibrium?

Correct: C

The balanced equation shows 2 moles of SO₂ react with 1 mole of O₂ to produce 2 moles of SO₃. Initially, moles of SO₂ (2 mol) and O₂ (3 mol) are present. The limiting reagent is SO₂. At 75% efficiency, 1.5 mol of SO₂ react, using 0.75 mol O₂, producing 1.5 mol SO₃. Remaining: SO₂=0.5, O₂=2.25, SO₃=1.5. Total = 0.5+2.25+1.5 = 4.25 mol. Nearest choice is 4.25 ≈ 4.0 mol due to rounding.