Assuming that CO2 is an ideal gas, we use the equation PV=nRT to determine n and multiply by molar mass to obtain mass. At STP, temperature is equal to 298.15 K and pressure is 1 atm.
PV = nRT
n = PV / RT
n = 1 (2.80) / 0.08205 (298.15)
n = 0.1145 mol
mass = 0.1145 ( 44.01 ) = 5.04 g CO2
