## Q. 19.2

Using Molecular Scenes to Examine Buffers

Problem The molecular scenes below represent equal-volume samples of four $HA/A^−$ buffers (Fig 19.2). (HA is blue and green, and $A^−$ is green; other ions and water are not shown.)

(a) Which buffer has the highest pH?

(b) Which buffer has the greatest capacity?

(c) Should you add a small amount of concentrated strong acid or strong base to convert sample 1 to sample 2 (assuming no volume change)? ## Verified Solution

Plan The molecular scenes show varying numbers of weak acid molecules (HA) and the conjugate base $(A^−)$. Because the volumes are equal, the scenes represent molarities as well as numbers. (a) As the pH rises, more HA loses its $H^+$ and becomes $A^−$, so the $[A^−]/[HA]$ ratio will increase. We examine the scenes to see which has the highest ratio. (b) Buffer capacity depends on buffer-component concentration and ratio. We examine the scenes to see which has a high concentration and a ratio close to 1. (c) Adding strong acid converts some $A^−$ to HA, and adding strong base does the opposite.

Comparing the $[A^−]/[HA]$ ratios in samples 1 and 2 tells which to add.

Solution (a) The $[A^−]/[HA]$ ratios are as follows: For sample 1, $[A^−]/[HA] = 3/3 = 1$. Similarly, sample 2 = 0.5; sample 3 = 1; sample 4 = 2. Sample 4 has the highest pH because it has the highest $[A^−]/[HA]$ ratio.
(b) Samples 1 and 3 have a $[A^−]/[HA]$ ratio of 1, but sample 3 has the greater capacity because it has a higher concentration.
(c) Sample 2 has a lower $[A^−]/[HA]$ ratio than sample 1, so you would add strong acid to sample 1 to convert some $A^−$ to HA.