What is the total pressure when 1.00 L of methane (CH4) at STP and 4.00 L of oxygen at STP are mixed in a 12.0-?L container? If the mixture is ignited and the methane completely combusts, producing carbon dioxide (CO2) and water (H2O), calculate the mass percent, the mole fraction, and the partial pressure of CO2 in the container after cooling back down to the initial temperature.