A 2.85 g sample containing only MgO and CuO is placed in a 3.00 L container. The container is filled with CO2 to a pressure of 740 torr at 20 degrees C. After the reaction has gone to completion, the pressure inside the flask is 390 torr at 20 degrees C. What is the mass percent of MgO in the mixture? Assume that only the MgO reacts with CO2.