Option (B) closer to the end where HCl was introduced is correct because, according to Graham's Law of Diffusion, The Rate of diffusion is indirectly proportional to 1/√(Molar. mass).
The ammonium chloride (NH₄Cl) will form primarily in the middle of the glass tube, where the two gases (NH₃ and HCl) are first able to react with one another. This is because the gases travel through the tube at different velocities, so the reaction will occur where the gases first meet.
As the reaction progresses, the ammonium chloride will continue to form until the reaction is complete and the tube is filled with the solid product.
The reaction of the two gases is reversible, meaning that the NH₄Cl can also break down back into NH₃ and HCl. However, as the gases continue to travel through the tube, the reaction rate of the NH₄Cl breaking down is much slower than the rate at which it is formed.
Therefore, the majority of the ammonium chloride will form in the middle of the tube where HCl is introduced.
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