PV=nRT
you are looking for volume, you have P (remember it has to be in atm)
R = constant, T is temp (in K)
you don't have n which is moles but you have a density 0.7025 g/mL.
If you use 1.00 mL of the octane, how many grams of the octane are you using? how many moles?
you should now have P, n, R and T, find V