[sodium.dioxid]

In calorimetry, do we apply the equation q=mCΔT to the reactants or to the products? I am confused because the content does not stay the same, it changes (from reactants to products).

[Borek]

Depending on what you are doing you can apply it to each separately or to the sum, you just have to watch what is m and what you use for C. For not too concentrated water solutions we usually assume 4.18 J·g^-1·K^-1.