Molecular weight = 2 × Vapour Density = 2 × 14 = 28. "X has volume half that of oxygen" implies the mole ratio is 1:2 if n=1 or 2:2 if n=2. Assuming formula XO₂, the mass would be X + 32, not 28. Assuming formula X₂O, the mass is 2X + 16 = 28, so 2X=12, X=6. [cite: 1, 1487, 1496]