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. degree C. After the reaction has gone to completion, the pressure inside the flask is 390. torr at 20. degree C. What is the mass percent of MgO in the mixture? Assume that only the MgO reacts with CO2

Answer 1

Let me give you some hints and you take it from there.
Use PV = nRT and calculate moles CO2 in the container initially.
Use PV = nRT and calculate moles CO2 in the container after the reaction is complete.
The difference between before and after should be the moles CO2 that reacted to give MgCO3 with the MgO. That should be equal to moles MgO that reacted (1:1 ratio if you write the equation), change that to grams, and calculate percent from that.

Answer 2

89.8%