At a certain temperature and pressure, one liter of co2 gas weighs 1.15 g. what is the mass of one liter of n2 gas at the same temperature and pressure?

Set up a proportion os V1/n1 = V2/n2. V is the volume, n is the amount in MOLES, not grams. Convert the CO_2 to moles, then solve and find that the amount of N_2 should be the same amount of moles. Then use the molar mass of N_2 (28.02 grams/mole) to convert that amount of moles into grams. That's your answer.