Which of the following gases are arranged in increasing order of diffusion rate? [H = 1 .0; C = 12; N = 14; O = 16; S = 32]

Concept (Graham's Law of Diffusion): The rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Heavier gases diffuse more slowly, and lighter gases diffuse faster.

So "increasing order of diffusion rate" means the molar masses must go from largest to smallest.

Molar masses:

• SO₂ = 32 + 32 = 64
• O₂ = 32
• NH₃ = 14 + 3 = 17
• H₂ = 2
• H₂S = 2 + 32 = 34
• NH₂ = 16 (this is probably a misprint for NH₃)
• CO₂ = 44
• N₂O = 44
• NO₂ = 46
• N₂ = 28

Checking each option:

• A. SO₂ (64), O₂ (32), NH₃ (17), H₂ (2). The molar masses decrease, so the diffusion rates increase. ✔
• B. H₂S (34), NH₂ (16), O₂ (32), SO₂ (64). The masses go 34, 16, 32, 64, which is not a steady decrease. ✘
• C. CO₂ (44), N₂O (44), O₂ (32), SO₂ (64). The masses go 44, 44, 32, 64, which is not a steady decrease. ✘
• D. NH₃ (17), NO₂ (46), N₂ (28), CO₂ (44). The masses go 17, 46, 28, 44, which is not a steady decrease. ✘

Answer: Only option A has the gases ordered from heaviest to lightest, which means from slowest to fastest diffusion.

Tip: For "increasing rate of diffusion" questions, just check that the molar masses decrease along the list. Heavy gas means slow, and light gas means fast. Remember that H₂ is the fastest-diffusing gas of all.