Question

What is the function of calorimeter in Hess's Law experiment?

Answer 1

It is to find the heat absorbed or released for any simple reaction so you could then add them to the final reaction you are looking for.

Answer 2

Hess’s Law holds that if a reaction can be expressed as the sum of two other
reactions, then the enthalpy change for the first reaction will also equal the sum
of the enthalpy changes for the other two reactions. Today you will deal with the
following three reactions. Notice that the third can be expressed as the sum of
the first two.
NaOH(s)  NaOH(aq)
NaOH(aq) + HCl(aq)  H2O(l) + NaCl(aq)
NaOH(s) + HCl(aq)  H2O(l) + NaCl(aq)