Graham's Law of Diffusion

Steps:

• Ammonia (NH₃, molar mass 17 g/mol) and hydrogen chloride (HCl, molar mass 36.5 g/mol) start at equal pressure from opposite ends.
• Lighter NH₃ diffuses faster than heavier HCl, per Graham's law: rate ∝ 1/√molar mass.
• NH₃ travels farther, so they meet closer to HCl end, forming white NH₄Cl solid there.
• Statements 1, 2, and 3 all explain this: faster diffusion and lower mass of NH₃.

Why A is correct:

• Includes all true statements; identical 2 and 3 both state NH₃'s lower molar mass, directly causing faster diffusion (Graham's law).

Why the others are wrong:

• B omits statement 3, which is valid.
• C ignores mass statements, missing root cause.
• D excludes statement 1, ignoring diffusion rate observation.

Final answer: A