AP Chemistry · Exam guide
AP Chemistry Acid-Base and Buffer Question Explained: pH, Ka and the Henderson-Hasselbalch Equation
Acid-base questions are among the most common on the AP Chemistry free-response section, and they combine a calculation with an explanation at the particle level. The numbers below are made up for practice, using the Ka of acetic acid, 1.8 times 10 to the minus 5.
The weak acid pH calculation
Find the pH of a 0.100 M solution of acetic acid.
- Ka = [H+][A-] / [HA] = x^2 / (0.100 - x).
- Because Ka is small, assume x is much smaller than 0.100, so x^2 / 0.100 = 1.8 times 10 to the minus 5.
- x = 0.00134 M, which is [H+].
- pH = -log(0.00134) = 2.87.
- Check the assumption: 0.00134 / 0.100 is 1.3 percent, which is under 5 percent, so the approximation is acceptable.
The buffer calculation
A buffer is made from 0.100 M acetic acid and 0.200 M acetate. Find its pH. The pKa is 4.74.
- Henderson-Hasselbalch: pH = pKa + log([A-] / [HA]).
- pH = 4.74 + log(0.200 / 0.100) = 4.74 + 0.30 = 5.05.
Now 0.010 mol of HCl is added to 1.00 L of the buffer. The H+ reacts with acetate, so acetate falls to 0.190 M and acetic acid rises to 0.110 M. The new pH is 4.74 + log(0.190 / 0.110) = 4.98. The pH moved by only 0.07 units.
Explaining how a buffer works
It states what a buffer does, but not how.
It names both buffer components, says which reacts with which, and ties that to the pH.
Common mistakes
- Using the buffer equation for a solution with only a weak acid.
- Using concentrations in the ratio when volumes were mixed, without recalculating them after dilution.
- Skipping the check of the 5 percent approximation.
- Putting the base over the acid backward in the Henderson-Hasselbalch ratio.
- Giving a pH to too many digits. Two decimal places is about right.
Frequently asked questions
How do I know if something is a buffer?
It has a weak acid and its conjugate base (or a weak base and its conjugate acid) in similar amounts.
When can I use the 5 percent rule?
When x is less than 5 percent of the starting concentration. Compute x, then check.
What is the pKa?
The negative base-10 logarithm of Ka. When the acid and its conjugate base are equal, the pH equals the pKa.
Why does a buffer have a limit?
Added acid or base uses up one of the two components. When one runs out, the pH changes quickly.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Chemistry course ends with practice items and an Exam focus box, and every unit has a ten-question practice set with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Chemistry course Free lessons and lesson list
Other free-response guides:
- AP Chemistry FRQ Guide: Long and Short Questions, Units and Particle Explanations
- AP Chemistry Equilibrium Question Explained: ICE Tables, Q Versus K and Le Chatelier
- AP Chemistry Particle-Level Explanations: How to Explain Trends, Boiling Points and Bonding
- AP Chemistry Lab and Data Questions Explained: Graphs, Beer's Law and Sources of Error
AP is a registered trademark of the College Board, which is not affiliated with and does not endorse this site. This guide is our own explanation of the question types and scoring. For the official scoring guidelines, see the College Board.