Molar Mass and Gas Volume Equivalence

Steps:

• Molar mass of water (H₂O) is 18 g/mol, so one mole has mass 18 g.
• At standard room temperature and pressure (RTP), one mole of ideal gas occupies ~24 L.
• Steam (water vapor) follows the ideal gas law, so 24 L of steam contains one mole.
• Mass of this steam equals the molar mass of water, 18 g.

Why B is correct:

• By Avogadro's law, equal volumes of gases at same T and P contain equal moles; 24 L at RTP holds 1 mol of H₂O gas (18 g).

Why the others are wrong:

• A: 24 mL liquid water has density ~1 g/mL, so mass 24 g ≠ 18 g.
• C: One H₂O molecule has mass 18 u ≈ 3 × 10⁻²³ g, not 18 g.
• D: 2 mol H₂ (4 g) + 1 mol O₂ (32 g) = 36 g, mass for 2 mol H₂O.

Final answer: B