| Date | May 2018 | Marks available | 1 | Reference code | 18M.3.sl.TZ1.1 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Draw | Question number | 1 | Adapted from | N/A |
Question
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.
Part of this molecule is hydrophilic (bonds readily to water) and part hydrophobic (does not bond readily to water). Draw a circle around all of the hydrophilic part of the molecule.
When a small amount of palmitic acid is placed in water it disperses to form a layer on the surface that is only one molecule thick. Explain, in terms of intermolecular forces, why this occurs.
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.
The solution of palmitic acid had a concentration of 0.0034 mol dm−3. Calculate the number of molecules of palmitic acid present in the 0.050 cm3 drop, using section 2 of the data booklet.
Assuming the sudden change in gradient occurs at 240 cm2, calculate the area, in cm2, that a single molecule of palmitic acid occupies on surface of the water.
If you did not obtain an answer for (b)(ii) use a value of 8.2 × 1016, but this is not the correct answer.
Markscheme
Must cut CH2–CO bond AND enclose all of the –COOH group.
[1 mark]
Any two of:
–COOH/CO/OH/carboxylate/carboxyl/hydroxyl/hydroxy group forms hydrogen bonds/H-bonds to water
London/dispersion/instantaneous induced dipole-induced dipole forces occur between hydrocarbon chains
hydrocarbon chain cannot form hydrogen bonds/H-bonds to water
strong hydrogen bonds/H-bonds between water molecules exclude hydrocarbon chains «from the body of the water»
Accept “hydrophilic part/group forms hydrogen bonds/H-bonds to water”.
Accept “hydrophobic section” instead of “hydrocarbon chain”.
Award [1 max] for answers based on “the –COOH group being polar AND the hydrocarbon chain being non-polar”.
[2 marks]
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]
amount of acid = «5.0 × 10–5 dm3 × 0.0034 mol dm–3» = 1.7 × 10–7 «mol»
number of molecules = «1.7 × 10–7 mol × 6.02 × 1023 mol–1 =» 1.0 × 1017
Award [2] for correct final answer.
Award [1] for “1.0 × 1020”.
[2 marks]
«area = \(\frac{{240{\text{ c}}{{\text{m}}^2}}}{{1.0 \times {{10}^{17}}}}\) » 2.4 × 10–15 «cm2»
[1 mark]
