Know that a calorimeter is used to determine ΔH at constant V

Calorimeters are devices used in thermodynamics to measure the heat energy exchanged during a chemical or physical process. They are particularly useful in determining the change in enthalpy (ΔH) of a system.

The statement "Know that a calorimeter is used to determine ΔH at constant V" means that a calorimeter is designed to measure the change in enthalpy at constant volume (V). In a constant volume calorimeter, the volume of the system remains constant throughout the process, allowing for the determination of ΔH under these specific conditions.

When using a calorimeter to determine the heat of combustion of a substance with oxygen, we can obtain the ΔH for the substance at constant pressure (c). This is because combustion reactions typically occur under atmospheric pressure, and a constant pressure calorimeter is commonly used to measure the heat changes associated with these reactions.

In a constant pressure calorimeter, the pressure remains constant throughout the process. This is achieved by using an open system or ensuring that the pressure inside the calorimeter is the same as the surrounding atmospheric pressure. By maintaining a constant pressure, the heat exchange can be accurately measured and used to determine the enthalpy change (ΔH) for the substance.

The option (c) ΔH for the substance at constant pressure aligns with the concept of using a calorimeter to determine the heat of combustion. It takes into account the fact that combustion reactions usually occur at constant atmospheric pressure and can be accurately measured in a constant pressure calorimeter.

The other options can be eliminated as follows:

• Option (a) ΔH for a constant mass of the substance is not necessarily true because the mass of the substance may change during the combustion process.

• Option (b) ΔH for the substance at constant temperature is not accurate because the temperature may change during the combustion process.

• Option (d) ΔH for the substance at constant volume is not applicable as the volume usually changes during the combustion process.

• Option (e) ΔH for the substance at constant product PV is not directly related to the use of a calorimeter in determining the heat of combustion.

In summary, a calorimeter is used to determine the change in enthalpy (ΔH) at constant volume (V). When using a calorimeter to measure the heat of combustion of a substance with oxygen, the ΔH for the substance can be determined at constant pressure (c). This is achieved using a constant pressure calorimeter, which allows for accurate measurement of the heat exchange during the combustion process.