The oxides of Group 2A (symbolized by M here) react with carbon dioxide according to the following reaction...

Question

MO(s) + CO2(g) MCO3(s)

A 2.80 g sample containing only MgO and CuO is placed in a 3.16 L container. The container is filled with CO2 to a pressure of 740. torr at 20.°C. After the reaction has gone to completion, the pressure inside the flask is 390. torr at 20.°C. What is the mass percent of MgO in the mixture? Assume that only the MgO reacts with CO2.

Answer 1

The problem is to find out how much of the original 2.8 g sample is MgO given the reaction of the sample with CO2 brings about a reduction in pressure.

Assume first that the volume of solids can be neglected compared to 3.16L. We can find the initial moles of CO2 in the container from n= PV/RT.
The moles of CO2 reacted are 350/740 x n. The
moles of MgO from the reaction is the same. So we can find the mass of MgO from
molwt MgO x 350/740 x n, and the mass percent follows directly.