Three ideal gases, Ammonia (NH3), Methane (CH4), and Oxygen (O2), are allowed to mix at 200 kPa and 400°C. The mixture contains 30 % NH3, 50 % CH4, and 20 % O2. The mixture is heated to 600°C. Calculate the change in the molar internal energy of the mixture for the heating process. You can assume the mixture is an ideal gas.