# When to include the mass of the dissolved substance in calorimetry problems?

In some calorimetry problems, in which a substance is dissolved in water and we want the heat of the reaction, the mass of the dissolved substance is included in the mass plugged into mcat but other times it isn't. When should I include the mass of the substance?

For example, in this problem the mass of $$\ce{NH4NO3}$$ is not added to the mass of water to find the enthalpy:

$$\pu{5.44 g}$$ of ammonium nitrate was added to $$\pu{150.0 mL}$$ of water in a coffee-cup calorimeter. The temperature changed from $$\pu{18.6 °C}$$ to $$\ce{16.2 °C}.$$ Calculate the enthalpy change for dissolving $$\ce{NH4NO3(s)}$$ in water in $$\pu{kJ mol^-1}.$$ Assume the solution has a specific heat capacity of $$\pu{4.18 J g^-1 K^-1}.$$

But in this problem the mass of $$\ce{CaCl2}$$ is added to the mass of water:

In a coffee cup calorimeter, $$\pu{2.6 g}$$ $$\ce{CaCl2(s)}$$ was dissolved in $$\pu{260 g}$$ of water at a combined initial temperature of $$\pu{23 °C}.$$ The final temperature was $$\pu{26.4 °C}.$$ Calculate the enthalpy change of the reaction.

I would really appreciate if someone could explain to me when to include the mass of the dissolved substance.