Gas diffusion follows Graham's law Steps:

• Confirm both H2S and PCl3 are gases at 90°C, exceeding their boiling points (-85°C and -61°C).
• Apply Graham's law: diffusion rate ∝ 1/√M, where M is molar mass.
• Calculate molar masses: H2S = 34 g/mol, PCl3 ≈ 137 g/mol.
• H2S has lower M, so higher diffusion rate, reaching detector first despite equal masses.

Why B is correct:

• B states PCl3 reaches second because H2S has lower M (34 vs. 137 g/mol), aligning with Graham's law where lower M yields faster diffusion.

Why the others are wrong:

• A: Both compounds are gases at 90°C, not one gas and one liquid.
• C: Incorrectly assigns H2S as first but repeats reason without specifying compound order.
• D: PCl3 has higher M than H2S, so slower diffusion.

Final answer: B