In combustion analysis: CaCl₂ tube absorbs H₂O and potash tube absorbs CO₂.
Mass increase of CaCl₂ = 0.15 g gives moles of H₂O = 0.15/18 = 0.00833 mol, so moles of H = 0.01667 mol and mass of H = 0.01667 g.
Mass increase of potash = 0.1837 g gives moles of CO₂ = 0.1837/44 = 0.00417 mol, so moles of C = 0.00417 mol and mass of C = 0.05 g.
Mass of O = 0.25 - 0.05 - 0.01667 = 0.18333 g.
Percentage of O = (0.18333/0.25) × 100 = 73.33% ≈ 73% (nearest integer).