| Date | May 2017 | Marks available | 2 | Reference code | 17M.3.sl.TZ2.2 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain | Question number | 2 | Adapted from | N/A |
Question
Disposable plastic lighters contain butane gas. In order to determine the molar mass of butane, the gas can be collected over water as illustrated below:
List the data the student would need to collect in this experiment.
Explain why this experiment might give a low result for the molar mass of butane.
Suggest one improvement to the investigation.
Markscheme
mass/m of lighter before AND after the experiment
volume of gas/Vgas «collected in the cylinder»
«ambient» pressure/P «of the room»
temperature/T
Accept “change in mass of lighter”.
Accept “weight” for “mass”.
Do not accept just “mass of lighter/gas”.
Accept “volume of water displaced”.
Do not accept “amount” for “volume” or “mass”.
[4 marks]
Any two of:
pressure of gas not equalized with atmospheric/room pressure
too large a recorded volume «of gas produces a lower value for molar mass of butane»
OR
cylinder tilted
difficult to dry lighter «after experiment»
OR
higher mass of lighter due to moisture
OR
smaller change in mass but same volume «produces lower value for molar mass of butane»
using degrees Celcius/°C instead of Kelvin/K for temperature
Accept “vapour pressure of water not accounted for” OR “incorrect vapour pressure of water used” OR “air bubbles trapped in cylinder”. Do not accept “gas/bubbles escaping «the cylinder»” or other results leading to a larger molar mass.
Accept “lighter might contain mixture of propane and butane”.
Do not accept only “human errors” OR “faulty equipment” (without a clear explanation given for each) or “mistakes in calculations”.
[2 marks]
record vapour pressure of water «at that temperature»
OR
equalize pressure of gas in cylinder with atmospheric/room pressure
OR
tap cylinder before experiment «to dislodge trapped air»
OR
collect gas using a «gas» syringe/eudiometer/narrower/more precise graduated tube
OR
collect gas through tubing «so lighter does not get wet»
OR
dry lighter «before and after experiment»
OR
hold «measuring» cylinder vertical
OR
commence experiment with cylinder filled with water
Accept “adjust cylinder «up or down» to ensure water level inside cylinder matches level outside”.
Accept “repeat experiment/readings «to eliminate random errors»”.
Accept “use pure butane gas”.
[1 mark]
