Equation for decomposition is as follow,
2 NH₄ClO₃ → 3 O₂ + 2 NH₄Cl
According to eq.
67.2 L (3 mole) off O₂ is produced by = 202.98 g (2 moles) NH₄ClO₃
So,
200 L of O₂ will be produced by = X g of NH₄ClO₃
Solving for X,
X = (202.98 g × 200 L) ÷ 67.2 L
X = 604.10 g of NH₄ClO₃