Question

How many moles of propane gas (`"C"_3"H"_8`) can be burned with 1 mole of oxygen gas at STP?

I know that `"C"_3"H"_8+5"O"_2rarr3"CO"_2+4"H"_2"O"` and I got `0.2"mol"`, but that seems too easy for the worksheet it was on. Is the fact that it's at STP just extra info to throw me off or am I missing something?

Answer 1

You are correct. 1 mole `O_2` consumes 1/5th mole (0.20 mole) propane.

Explanation:

The equation as written is 'assumed' to be at STP if written using 'Standard State' formulas published in Thermodynamics Properties Tables. Of course STP represents (in most college texts) `0^oC` & 1Atm Pressure. The tables, however, specify standard thermodynamic values which are at `25^oC`. This is an issue if calculating volume values as 1 mole of gas at `0^oC` is 22.4L/mole but slightly larger at `25^oC` at 24.45L/mole.

`V_25` = (298/273)(22.4 L/mole) = 24.45 L/mole

For 'mass' calculations, it does not matter. mass at `0_oC` = mass at `25^oC`.

Your use of the equation reaction ratios in the balanced equation are spot on correct.