Hi, you need to begin this question (like most chemistry questions) with writing down the chemical equation and balancing it.
C3H8 + 5O2 → 3CO2 + 4H2O
We can see that for every 1 mol of propane, there are 5 mols are oxygen.
If we have 5 grams of propane, how can we convert that into mols? We need to use the molar mass of propane, which is 44.1 g / mol. (12 g/mol x 3 + 1 g/mol x 8)
To convert to mols of propane: n (mols) = mass/molar mass -> 5/44.1 which equals 0.011 mols.
We said that for every 1 mol of propane, there are 5 mols of oxygen, correct? Therefore, 0.011 mols x 5 = 0.57 mols of oxygen needed to react with 5 grams of propane.
To convert 0.055 mols of oxygen to grams, we use the same equation. n = Mass/Molar Mass
0.57 = g/32, solve for the mass in grams of oxygen and we come to: 18.2 grams of Oxygen.
Good luck! I am a chemistry tutor if you are ever in need of help.