Thermochemistry is the study of heat and energy associated with a chemical reaction or a physical transformation. A reaction may absorb or release energy. Measurement of heat changes are carried out in vessels called calorimeters. During measurement of heat changes, the calorimeter also absorbs some heat; this amount of heat should also be known. It is called calorimeter constant or water equivalent of calorimeter. To determine the water equivalent of calorimeter, known volume of hot water at a specified temperature is added to known volume of water contained in the calorimeter at room temperature. Since energy is conserved, the heat taken by calorimeter and cold water should be equal to heat given by hot water.
This video explains how to determine (A) Water equivalent of calorimeter and (B) Enthalpy of dissolution of given salt.