AP Chemistry · Unit 8: Acids and Bases
8.9 Henderson-Hasselbalch Equation
In short
You must understand the relationship between the pH of a buffer, the pKa of the acid, and the concentration ratio of the conjugate acid-base pair, as described by the Henderson-Hasselbalch equation. You should recognize that adding small amounts of acid or base to a buffered solution does not significantly change the [A−]/[HA] ratio, meaning the pH change is much less than it would be in an unbuffered solution.
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What else do you need to know about Henderson-Hasselbalch Equation?
The AP Exam explicitly excludes the computation of the change in pH resulting from the addition of an acid or a base to a buffer, as well as the derivation of the Henderson-Hasselbalch equation.
Can you do these?
- I can explain how the pH of a buffer is related to the pKa and the concentration ratio of the conjugate acid-base pair using the Henderson-Hasselbalch equation, and understand why adding small amounts of acid or base does not significantly change the pH.
How is Henderson-Hasselbalch Equation tested on the exam?
This topic is assessed under the Mathematical Routines skill category (Skill 5.F). You may be asked to calculate, estimate, or predict an unknown quantity from known quantities by selecting and following a logical computational pathway and attending to precision (e.g., performing dimensional analysis and attending to significant figures). Examiners will not assess the computation of the change in pH resulting from the addition of an acid or a base to a buffer, nor will they assess the derivation of the Henderson-Hasselbalch equation.
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Henderson-Hasselbalch Equation: common questions
What do you need to know about Henderson-Hasselbalch Equation for AP Chemistry?
You must understand the relationship between the pH of a buffer, the pKa of the acid, and the concentration ratio of the conjugate acid-base pair, as described by the Henderson-Hasselbalch equation. You should recognize that adding small amounts of acid or base to a buffered solution does not significantly change the [A−]/[HA] ratio, meaning the pH change is much less than it would be in an unbuffered solution.
How is Henderson-Hasselbalch Equation tested on the AP Chemistry exam?
This topic is assessed under the Mathematical Routines skill category (Skill 5.F). You may be asked to calculate, estimate, or predict an unknown quantity from known quantities by selecting and following a logical computational pathway and attending to precision (e.g., performing dimensional analysis and attending to significant figures).
How do I know I have mastered Henderson-Hasselbalch Equation?
Check you can do each of these: explain how the pH of a buffer is related to the pKa and the concentration ratio of the conjugate acid-base pair using the Henderson-Hasselbalch equation, and understand why adding small amounts of acid or base does not significantly change the pH.
Where does Henderson-Hasselbalch Equation fit in AP Chemistry?
Henderson-Hasselbalch Equation is topic 8.9 in Unit 8: Acids and Bases of AP Chemistry.
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