The scenes below depict solutions of the same HA/A^- buffer...

The scenes below depict solutions of the same HA/A$^{-}$ buffer (with other ions and water molecules omitted for clarity). (a) Which solution has the greatest buffer capacity? (b) Explain how the pH ranges of the buffers compare. (c) Which solution can react with the largest amount of added strong acid?

Key Concepts

Buffer Capacity

Buffer capacity is a measure of how much acid or base a buffer solution can neutralize before a significant change in pH occurs. It depends on the absolute concentrations of the weak acid and its conjugate base, with higher concentrations generally providing a greater capacity to resist pH changes.

Buffer pH Range

The effective pH range of a buffer is typically centered around the pKa of the weak acid and extends approximately one pH unit above and below that value. This range reflects the concentrations in which the conjugate acid/base pair can most effectively resist added acids or bases.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation, expressed as pH = pKa + log([A?]/[HA]), links the pH of an acid-base buffer system to the ratio of the concentrations of the conjugate base to the weak acid. It is a fundamental tool for understanding and predicting the behavior of buffer solutions.

Acid-Base Neutralization in Buffers

In buffer systems, the ability to neutralize added strong acid is determined by the availability of the conjugate base, which reacts with the acid to form the weak acid. This reaction, and the buffer’s resulting neutralizing power, directly depends on the amount of the conjugate base present in the buffer.

Transcript

00:01 All right, i've drawn beforehand here, the four beakers filled with the solution.

00:06 And i've represented ha by both red and blue, and the a minus is the only red.

00:15 So if we count it up here, the first one, h .a.

00:19 Equals seven, and a minus equals three.

00:22 And this one, h .a.

00:24 Equals three, a minus equals seven.

00:26 The third one, h .a.

00:28 Equals 5, a minus equals 5.

00:30 Fourth one, h .a.

00:31 Equals 3 and a minus equals 3.

00:36 And so to answer this first question, which solution has the greatest buffer capacity? we can take a look here at the henderson -hasselbach equation.

00:46 And when a -minus equals ha, then the ph equals the p -k -a.

00:52 And the reason for that is because when you take the log of 1, so if these numbers, a minus and h .a.

00:58 Are the same.

01:00 Then that's 1 over 1 essentially.

01:03 And if you take the log of 1, that equals 0.

01:05 And then the p .a equals the ph.

01:09 So if we go back here and take a look at these, the third and the fourth one right here, they have even numbers.

01:16 And so they would have the greatest buffer capacity.

01:20 Now the question is, which one out of the two has the better buffer capacity? and the answer is the third one, because it has a greater concentration inside of it.

01:34 And so since there's more molecules, there's more molecules available to neutralize any added acid or base...

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