The principle of uniformitarianism describes the fact that the geological processes used to occur in the past were in the same manner and with the same intensity that as they occur in the present on the planet Earth. Therefore, uniformity in the geological processes exhibit from the unknown and unobservable past to present scenario of Earth. The present is the key to the past is the slogan of the principle of uniformitarianism.
The final volume of the Oxygen O2 would simply be equivalent to the overall volume of the final mixture. This is because gases would occupy every space of its container. So in this case, the volume of CO2 and O2 would be the same and it would be 30 L. What would be different is the partial pressures of each.
The mass of the atom would change. With no atoms being there then the mass would possibly change. With no neutrons then the mass of the atom will decrease