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2024 O Level Chem P1 Q7 — Diffusion / Kinetic Particle Theory Solution

Diffusion / Kinetic Particle Theory 2024 O Level Pure Chemistry · Paper 1 · Q7

Question

In experiment 1, concentrated HCl(aq) is placed at one end of a tube, and concentrated NH₃(aq) is placed at the other. Gaseous HCl and NH₃ diffuse from each end. When the gases meet, they react and a white ring of NH₄Cl(s) appears. The most common isotope of hydrogen is ¹₁H. Hydrogen has another isotope called deuterium which has the symbol ²₁H and is given the symbol D. In experiment 2, using the same tube and temperature, the NH₃ is replaced with ND₃. The ND₃ and HCl react to form NHD₃Cl(s). Which diagram shows the position of the white ring of NH₄Cl and NHD₃Cl in the two experiments? Question diagram

Question diagram

Correct answer: A

Video Solution

Explanation

Answer: A. HCl (\(M_r\) = 36.5) is heaviest; both \(\text{NH}_3\) (\(M_r\) = 17) and \(\text{ND}_3\) (\(M_r\) = 20) are lighter, so both diffuse faster than HCl. The white ring forms where the gases meet, which is closer to the slower (heavier) gas — i.e., closer to the HCl end.

\(\text{NH}_3\) is lighter than \(\text{ND}_3\), so it diffuses fastest and meets HCl earliest. \(\text{NH}_4\text{Cl}\) ring forms most leftward (closest to HCl). \(\text{NHD}_3\text{Cl}\) forms slightly to the right of the \(\text{NH}_4\text{Cl}\) position, but still on the HCl half. Option A shows both rings on the HCl side, with \(\text{NH}_4\text{Cl}\) further left than \(\text{NHD}_3\text{Cl}\) — matching the physics.

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