We need to calculate the enthalpy change of a certain reaction, usually acid-base, acid-carbonate, or dissolution reactions. So we take the two substances and mix them together and record the temperature change. Using Q=mcT, we calculate the amount of thermal energy given off or taken in by the reaction, and then using H=Q/n we find the enthalpy change of the reaction. However, the confusion is what value of n (number of moles) to use.
Examples of reactions:
HCl + NaOH -> H20 + NaCl (here n is the mol of any one of the reactants or products, no confusion.)
2HNO3 + Na2CO3 -> H2O + 2NaNO3 (here teacher says take mol of H2O, because it is the one carrying off the thermal energy. Or take it as the mol of Na2CO3, because that is the reactant that is changing, not the acid.)
H2SO4 + KOH -> 2H2O + K2SO4 (here following the same rule as above, taking the number of mol of water, is for some reason wrong.)
Is there a general rule or concept that allows you to figure out exactly which number of mol to use?