Can you help me with this problem on group 2A metal reaction

Question

The oxides of the Group 2A metals (symbolized by M) react with carbon dioxide in the following reaction

 

MO(s)+CO2(g) = MCO3

 

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

Answer 1

How many moles of CO2 have been used in the reaction?  It can be determined by the change in pressure from 740 torr to 390 torr by using PV=nRT and solving for n (moles) or by simply taking the difference in pressure (350 torr) and converting to moels.  One can also assume that MgO is the only oxide that reacted since CuO is NOT a group 2 oxide.

moles CO2 initially present = (740 torr)(3.00L)/(62.36 Ltorr/Kmol)(293K) = 0.122 moles 

moles CO2 at end of reaction = (390 torr)(3.00L)/(62.36)(293) = 0.0640 moles

moles CO2 used in reaction = 0.122 - 0.064 = 0.058 moles CO2 used

From balanced equation, 1 mole CO2 ==> 1 moles MO

moles MgO = 0.058 moles

mass of MgO = 0.058 moles x 40.3 g/mole = 2.34 g

Mass % MgO = 2.34 g/2.85 g (x100%) = 82.1%