[Lilly5670]

A sample of propane (C3H8) is placed in a closed vessel together with an amount of O2 that is 2.50 times the amount
needed to completely oxidize the propane to CO2 and H2O at constant temperature. Calculate the mole fraction of the
each component in the resulting mixture after oxidation assuming that H2O is present as gas.

since not very much info is given I started to write a balanced formula and came up  with:\
2C3H8 + 10 O2 ---> 6CO2 + 8H2O but I don't know if I should multiply the amount of O2 by 2.5
I don't think I'm doing this right can someone please help? Thanks

[Schrödinger]

The equation which you've written is one based on the amount of oxygen required to completely oxidize the propane. Assuming that this reaction goes to completion (quantitative conversion), how much of O₂ is left now? Whatever's there in excess, carry over separately, without including in the reaction and confusing yourself, because it is not possible to balance the reaction if you have the excess incorporated in your stoichiometric coefficient.

[AWK]

2C3H8 + 10 O2 ---> 6CO2 + 8H2O
correct balancing! - all coefficients divisible by 2