On a 7 day wilderness expedition you'll need to heat 3.0kg of water to the boiling point each day. The air temperature will average 25°C.

Question

You have available canisters of compressed propane C3H8 fuel. Each canister has 50. g of propane in it. What is the minimum number of fuel canisters you must bring? The standard heat of formation of propane at 25°C is −103.8/kJmol.

Canisters = ________

Answer 1

3 kg/day x 7 days = 21 kg water

To find the heat needed, used q = mC∆T

∆T = 100º - 25º = 75º

q = (21 kg water)(4.184 kJ/kgº)(75º) = 6590 kJ needed

molar mass propane = 44.1 g / mol

50 g propane / tank x 1 mol / 44.1 g = 1.13 moles propane / tank

1.13 mols propane / tank x 103.8 kJ / mol = 118 kJ / tank

6590 kJ x 1 tank / 118 kJ = 55.8 tanks = 56 canisters minimum number

Answer 2

Hi Marissa B.,

Sure you COULD take your formation rxns and heats for propane, water, and CO2, and add them up appropriately to get the combustion rxn (and its enthalpy change!). And then heat the requisite water from RT to the boiling point.

BUT -- you would still need to boil it a while to kill Giardia cysts, I think, or maybe use iodine.

If you just want to purify the water, filtration apparatus is available, cheap, and works a treat (with appropriate care in operation -- you have to be careful never to get "dirty" water on the "clean" end of your filter cartridge or on your clean cup). Removes everything down to virus size.

When you're out for 7 days in the wilderness, mass of your supplies matters a great deal. With propane, you get heat, sure, but then must 'port the empties out again ("Leave No Trace").

Were you thinking of backpacking the Hundred Mile Wilderness (on the Appalachian Trail) in central Maine, by any chance? Go for it!

-- Cheers, --Mr. d.