Question 18M.3.sl.TZ1.1b.i
| Date | May 2018 | Marks available | [Maximum mark: 2] | Reference code | 18M.3.sl.TZ1.1b.i |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Suggest | Question number | b.i | Adapted from | N/A |
Palmitic acid has a molar mass of 256.5 g mol−1.
The apparatus in the diagram measures the surface pressure created by palmitic acid molecules on the surface of water. This pressure is caused by palmitic acid molecules colliding with the fixed barrier. The pressure increases as the area, A, available to the palmitic acid is reduced by the movable barrier.
When a drop of a solution of palmitic acid in a volatile solvent is placed between the barriers, the solvent evaporates leaving a surface layer. The graph of pressure against area was obtained as the area A was reduced.
Suggest why there is a small increase in the surface pressure as the area is reduced to about 240 cm2, but a much faster increase when it is further reduced.
[2]
Above about 240 cm2:
greater collision frequency/collisions per second between «palmitic acid» molecules and the barrier «as area reduced»
At less than about 240 cm2:
molecules completely cover the surface
OR
there is no space between molecules
OR
force from movable barrier transmitted directly through the molecules to the fixed barrier
OR
«palmitic acid» molecules are pushed up/down/out of layer
For both M1 and M2 accept “particles” for “molecules”.
For M1 accept “space/area between molecules reduced” OR “molecules moving closer together”.
[2 marks]