Diffusion in gases
3 marks · NormalIn a laboratory experiment, students observe the diffusion of two gases through the air along a glass tube: ammonia (NH₃) is released at one end and hydrogen chloride (HCl) at the other. A white ring of ammonium chloride (NH₄Cl) forms 26 cm from the ammonia end and 14 cm from the hydrogen chloride end after several minutes.
Use the information to:
Calculate the total length of the glass tube.
Explain, using the particle model, why the white ring forms closer to the hydrogen chloride end than to the ammonia end.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 40 cm
(b) Ammonia particles are lighter and move faster than hydrogen chloride particles, so they travel further in the same time. The white ring forms where the two gases meet, which is closer to the slower-moving hydrogen chloride end. · must show reference to particle speed due to mass, connection to position of the white ring
Common mistakes: adding distances incorrectly; reversing which gas moves faster
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Adds the two distances (26 cm + 14 cm) to get total tube length.
M2 (b) · 1 mark · States/ammonia particles are lighter (smaller mass) and move faster OR hydrogen chloride particles are heavier and move slower.
A1 (b) · 1 mark · Explains that the ring forms where the gases meet, closer to the end with the slower gas (hydrogen chloride).
Worked solution
a) To find the total length of the tube, add the distances from both ends to where the ring appears:
So, the tube is 40 cm long.
b) The particle model explains that gas particles move randomly. Ammonia particles are lighter than hydrogen chloride particles, so they move faster and travel further in the same time. The white ring forms where NH3 and HCl meet and react, so it forms closer to the slower-moving HCl end. This shows that ammonia moves faster than hydrogen chloride in the same time.