CHM 2046 Lecture Notes - Lecture 1: Ph Meter, Sodium Hydroxide, Titration
Document Summary
The stoichiometric point is where the moles of oh- added equal the moles of h3o+ in the sample. It occurs at the steepest point in your graph, as seen in figure 18. 2. This point on the curve is known as the inflection point, which is a calculus term for the point where dy/dx (derivative or slope) is at a maximum, and where d2y/dx2 = 0 (the 2nd derivative is zero). There is an equilibrium expression for ha + h2o h3o+ + a- as follows: Ka = [h3o+][a-] / [ha] and pka = -log(ka) At half the volume (v/2) to the stoichiometric point (v), half of the acid has been neutralized, so [a-] = [ha] = 0. 5 [ha]0 at that point. These two values then cancel from ka, therefore ka = [h3o+] at v/2. Also at v/2, pka = -log(ka) = -log[h3o+] = ph, where ph is read at v/2 from your graph.
