According to the ideal gas law, a 10.0 mol sample of methane gas in a 0.815 L container at 503 K should exert a pressure of 506 atm. By what percent does the pressure calculated using the van der Waals' equation differ from the ideal pressure? For CH4 gas, a = 2.25 L2atm/mol2 and b = 4.28E-2 L/mol. %