1. Balanced Chemical Equation:

First, write the balanced chemical equation for the reaction:

N₂(g) + 3H₂(g) → 2NH₃(g)

2. Mole Ratio:

The balanced equation tells us that 1 mole of N₂ reacts to produce 2 moles of NH₃.

3. Calculation:

Since we know we have 0.40 moles of NH₃, we can use the mole ratio to find the moles of N₂ that reacted:

(0.40 moles NH₃) * (1 mole N₂ / 2 moles NH₃) = 0.20 moles N₂

Therefore, 0.20 moles of N₂ reacted.