AP Chemistry · Unit 8: Acids and Bases
8.7 pH and pKa
In short
The topic requires you to understand the relationship between the pH of a solution and the pKa of a weak acid or base to predict its predominant protonation state. You must know that when pH < pKa, the acid form predominates, and when pH > pKa, the base form predominates.
Last updated
What else do you need to know about pH and pKa?
The concept is applied to acid-base indicators, which change properties (like color) depending on their protonation state. You are expected to know how to select an appropriate indicator for a titration by ensuring its pKa is close to the pH at the equivalence point.
Can you do these?
- I can predict the protonation state of an acid or base by comparing the pH of a solution to the pKa of the acid.
- I can explain how acid-base indicators change properties based on their protonated versus deprotonated states in response to pH.
- I can select an appropriate acid-base indicator for a titration by choosing one with a pKa close to the pH at the equivalence point.
How is pH and pKa tested on the exam?
You are assessed on your ability to explain the relationship between predominant species and pH/pKa. Questions may require you to predict the relative concentrations of HA and A- given pH and pKa values. You may also be asked to select or justify the choice of an acid-base indicator for a specific titration based on equivalence point pH and indicator pKa. The suggested skill (2.D) indicates you may need to make observations or collect data from representations of laboratory setups or results, attending to precision.
Free practice questions with full explanations, and instant marking of your free-response answers.
pH and pKa: common questions
What do you need to know about pH and pKa for AP Chemistry?
The topic requires you to understand the relationship between the pH of a solution and the pKa of a weak acid or base to predict its predominant protonation state. You must know that when pH < pKa, the acid form predominates, and when pH > pKa, the base form predominates.
How is pH and pKa tested on the AP Chemistry exam?
You are assessed on your ability to explain the relationship between predominant species and pH/pKa. Questions may require you to predict the relative concentrations of HA and A- given pH and pKa values.
How do I know I have mastered pH and pKa?
Check you can do each of these: predict the protonation state of an acid or base by comparing the pH of a solution to the pKa of the acid; explain how acid-base indicators change properties based on their protonated versus deprotonated states in response to pH; select an appropriate acid-base indicator for a titration by choosing one with a pKa close to the pH at the equivalence point.
Where does pH and pKa fit in AP Chemistry?
pH and pKa is topic 8.7 in Unit 8: Acids and Bases of AP Chemistry.
Want help mastering this topic?
Work through pH and pKa one-on-one with an AP Guru tutor.
Book a free demo class