In a glass calorimeter, the pressure is typically assumed to be constant because glass calorimeters are open to the atmosphere. Since the calorimeter is not sealed, any change in volume due to a chemical reaction can be compensated by the exchange of gases with the surrounding environment. This exchange allows the pressure inside the calorimeter to remain relatively constant throughout the experiment. Therefore, the pressure term can be neglected in calculations involving glass calorimeters.
On the other hand, bomb calorimeters are designed to be sealed containers. They are typically used to measure the heat of combustion of substances, where the reactants are completely enclosed within the bomb calorimeter. In such a setup, the volume of the system remains constant because there is no exchange of gases or matter with the surroundings. The reaction occurs in a closed environment, maintaining a constant volume. As a result, the volume term can be neglected in calculations involving bomb calorimeters.
It's important to note that these assumptions simplify the calculations and allow for the determination of the heat transferred in the calorimetric experiments. However, in reality, there might be slight changes in pressure and volume, and corrections can be applied if those changes are significant or need to be taken into account for more accurate measurements.