Assume ideal gas and use the law:
p*V = n*R*T
p = 1.03*101.325 = 104.36475 kPa
V = 18.0 L
R = 8.314 J/(K*mole)
T = 21.7+273.15 = 294.85 K
n = 18*104.36475/(8.314*300.25) = 0.77 mole
Number of CO2(g) moles is 0.77