Chemistry IGCSE Science Worksheets, Questions and Revision

Cotton wool soaked in concentrated ammonia solution is placed at one end of a horizontal glass tube of length 100 cm, and cotton wool soaked in concentrated hydrochloric acid is placed at the other end, at the same moment. Ammonia gas (NH3, Mr = 17) and hydrogen chloride gas (HCl, Mr = 36.5) diffuse along the tube and react to form a white ring of ammonium chloride where they meet. Calculate the distance from the hydrochloric acid end at which the white ring forms.

1. Identify the Mr values: NH3 = 17 and HCl = 36.5.
2. Apply Graham's law to find the ratio of distances travelled: distance(NH3) / distance(HCl) = square root of (Mr(HCl) / Mr(NH3)) = square root of (36.5/17) = square root of 2.147 = 1.465.
3. Set up the total distance equation: distance(NH3) + distance(HCl) = 100 cm, with distance(NH3) = 1.465 x distance(HCl).
4. Substitute: 1.465 x distance(HCl) + distance(HCl) = 100, so 2.465 x distance(HCl) = 100.
5. Solve: distance(HCl) = 100 / 2.465 = 40.6 cm.
6. Final answer: the white ring forms 40.6 cm from the hydrochloric acid end (and 59.4 cm from the ammonia end), because the lighter ammonia gas diffuses faster and travels further.