Why is it impossible to determine the total amount of enthalpy in a substance?

The correct reasoning for this question lies in understanding what enthalpy is and the nature of thermodynamic measurements. Enthalpy is a state function, meaning it depends on the state of a system (like temperature and pressure) but not on how that state was reached. However, the total amount of enthalpy for a specific substance cannot be determined directly because enthalpy is defined in terms of a change in energy rather than an absolute value.

One key reason is that enthalpy values are relative; they are usually reported as changes in enthalpy from a standard state. This implies that we can only measure changes in enthalpy during chemical reactions or physical processes—not the total or absolute enthalpy of a substance itself.

Additionally, since the values of enthalpy depend on temperature and pressure, without a reference point, it makes it challenging to define an absolute enthalpy for a substance. Therefore, both of these factors—reliance on relative measurements and dependence on thermodynamic conditions—justify the inability to definitively determine the total amount of enthalpy in a substance.