To determine the number of moles of the gas, we need to relate moles to pressure, temperature and volume. To simplify things, we assume the gas is ideal so we use the ideal gas equation PV=nRT.
PV = nRT
n = PV / RT
n = 3.5 (9.0) / 0.08205 (74+273.15)
n = 1.1059 mol
