Composition diagrams, commonly known as "alpha plots," are often used to visualize the species in a solution of an acid or base as the $\mathrm{pH}$ is varied. The diagram for $0.100 \mathrm{M}$ acetic acid is shown here. (GRAPH CANNOT COPY) The plot shows how the fraction $[=\text { alpha }(\alpha)]$ of acetic acid in solution, $\left[\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\right] /\left(\left[\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\right]+\right.$
$\left.\left[\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}\right]\right),$ changes as the $\mathrm{pH}$ increases (blue curve). (The red curve shows how the fraction of acetate ion, $\mathrm{CH}_{3} \mathrm{CO}_{2}^{-},$ changes as the $\mathrm{pH}$ increases.) Alpha plots are another way of viewing the relative concentrations of acetic acid and acetate ion as a strong base is added to a solution of acetic acid in the course of a titration.
(a) Explain why the fraction of acetic acid declines and that of acetate ion increases as the pH increases.
(b) Which species predominates at a pH of 4, acetic acid or acetate ion? What is the situation at a pH of $6 ?$
(c) Consider the point where the two lines cross. The fraction of acetic acid in the solution is $0.5,$ and so is that of acetate ion. That is, the solution is half acid and half conjugate base; their concentrations are equal. At this point the graph shows the $\mathrm{pH}$ is 4.74 Explain why the $\mathrm{pH}$ at this point is 4.74