Disclaimer: thermo is my weakest subject so i cant explain it much. However ive been studying it some recently, so maybe I can give a little insight?

How you calculate entropy will depend a lot on what your system looks like. It sounds like you have a disconnect because you're trying to use the equation that assumes constant T when you don't have constant T. You need to manipulate the equation to make this work.

Is it adiabatic (no heat transfer. Are the walls of your system insulated?). Here the solvent is probably considered to be your "surroundings" (assuming the reaction is your "system") so you wouldn't be adiabatic. I'm just stabbing in the dark here.

Is it isothermal (can the temperature change? IE is it in contact with a temperature reservoir, likely in the form of a hot/cold bath). Seems like the answer is no.

Are the walls rigid? Can any PV (gas expansion) work be done?

What chemical reactions are taking place? What are you trying to calculate the entropy of? In what is this taking place in?

Etc.

The definition of entropy is dS = dQ/T

Depending on your system you can manipulate the variables and make substitutions. For example, for ideal gases and no change in pressure, dQ = dH. Then you get deltaS = deltaH/T (here we're assuming constant T)

Or more generally for gases, dQ = CpdT (Cp is the heat capacity at constant pressure). Here you get deltaS = Cpln (T2/T1).

Etc. There are lots of such ways to calculate deltaS. They typically require algebraic substitutions and some calculus (I skipped through the calculus bits, but they're not hard to show if you're familiar with basically integration).

I kind of babbled and threw out some equations, but we need a little more info to help you. At least I do, because I'm not well versed. If NY response confused you, don't worry. My professor's explanations confused (and continue to confuse) me too. What class is this for?