The thermodynamic properties of weak acid ionization reactions are determined. The thermodynamic properties are corresponding values of the absolute temperature (T), the weak acid equilibrium constant (K[subscript a]), the enthalpy of ionization (delta[subscript i]H[degrees]), and the entropy of ionization (delta[subscript i]S[degrees]). The enthalpy of ionization (delta[subscript i]H[degrees]) is determined from the enthalpy of neutralization of HCl(aq) and the enthalpy of neutralization of the weak acid by application of Hess's law; NaOH(aq) is the base. A datalogger and a temperature sensor (plus or minus 0.01 [degrees Celcius] sensitivity) are used to measure and plot the thermograms of the reactions. The calorimeter constant (C[subscript Cal) is determined by electrical heating of the post-reaction solution; procedure takes 10-20 s for each C[subscript Cal] determination. With NaOH(aq) as the limiting reagent, the post-reaction solutions consist of a weak acid and corresponding conjugate base allowing K[subscript a] to be determined from the measured pH of these solutions. The values of T, K[subscript a], and delta[subscript i]H[degrees] are used to calculate delta[subscript i]S[degrees] according to the equation: delta[subscript i]G[degrees] = -RT ln Ka = delta[subscript i]H[degrees] - Tdelta[subscript i]S[degrees]. The choice of H[subscript 3]PO[subscript 4](aq), a triprotic weak acid, provides an opportunity for students to predict and explain expected trends in K[subscript a], delta[subscript i]H[degrees], and delta[subscript i]S[degrees] prior to their determination. The multiconcept nature of this lab exercise makes it an ideal capstone laboratory exercise in general chemistry. (Contains 1 figure and 2 tables.)