Stoichiometry and ideal gas law determine O2 mass from pressure change Steps:

• Initial moles of NO2 = 6.9 g / 46 g/mol = 0.15 mol.
• Reaction: 2 NO2 → N2 + 2 O2; for z moles NO2 decomposed, O2 moles = z, total moles = 0.15 + 0.5z.
• Final total moles = 1.5 × initial moles = 1.5 × 0.15 = 0.225 mol (constant V, T).
• Solve 0.15 + 0.5z = 0.225 → z = 0.15 mol; O2 mass = 0.15 mol × 32 g/mol = 4.80 g. Why A is correct:
• Matches stoichiometry where 1 mol NO2 yields 1 mol O2 and pressure ratio 1.5:1 implies 50% mole increase per decomposed NO2 (ideal gas law: P ∝ n). Why the others are wrong:
• B: Doubles O2 mass, ignoring 1:1 NO2-to-O2 ratio.
• C: Approximates full initial mass (6.9 g), confusing total sample with O2.
• D: Assumes ~1.5 mol initial NO2 (69 g sample), misreading given mass. Final answer: A