December 2017 - January 2018

# A good problem relating to the gas laws

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Michael P Jansen and Justin Chan (AP Chemistry student), Crescent School, Toronto, Ontario

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Coleman Powermax fuel is a liquid fuel for camp stoves. Before this product was discontinued, it was sold in aluminium canisters. The liquid contents — under
pressure — are claimed to consist of

35% (m/m) propane (C3H8, boiling point –42 oC) and

65% (m/m) butane (C4H10, boiling point –1 oC).

In an attempt to verify the proportions (m/m) of this fuel, the following experiment was performed: the canister was cooled with saturated NH4Cl(aq) and lots of ice. Working in a fume hood, the chemists punctured the canister with a hammer and a nail.

This caused some gas to be emitted from the puncture hole. This emission of gas continued for ca 30 seconds. Then the remaining material in the canister — a clear liquid — was poured into a clear 2-L pop bottle and was allowed to boil away, leaving the pop bottle full of the gas form of the material; the bottle was then capped.

## Questions

1. Why was the canister cooled prior to being punctured? Why was the canister cooled in a bath of ice and NH4Cl(aq) instead of just plain ol’ ice and water?
2. Use the data above to determine the average molar mass of the gas collected.
3. How does your answer compare with the manufacturer’s claim with respect to the 35% (m/m) propane and 65% (m/m) butane? Provide a reasonable explanation for your answer. Provide a new data point for “pop bottle containing Coleman Powermax gas + lid” to support the gas mix claimed by the manufacturer.

“empty” soda bottle (contains air) + lid | 45.288 g
volume of soda bottle | 2.10 L
air pressure in the lab during the experiment | 763.3 mmHg
temperature of room during the experiment | 23.4 oC
soda bottle containing Coleman Powermax gas + lid | 47.819 g
mass of full Powermax cylinder before the experiment (with C3H8/C4H10 gas mixture) | 226.92 g
mass of empty Powermax cylinder after experiment | 83.25 g
approximation of components of air (v/v) | 78% N2, 21% O2, 1% Ar

Teacher’s note: Please note this is just the general outline of the procedure and details of the safety precautions are not listed. This data give an answer of almost 100% butane. Most of the propane "disappeared" as a gas when the cylinder was punctured. Careful reading of the up-front material reveals this clue. Email Jean Hein for a complete solution; jhein@uwaterloo.ca.

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