MCAT® ExamChemical and Physical Foundations of Biological SystemsMedium
A laboratory technician is preparing a buffer solution. They combine 0.10 mol of a weak acid (HA) with 0.10 mol of its conjugate base (A-) in 1.0 L of water. If the pKa of the weak acid is 4.75, what is the pH of the resulting buffer solution?
- A4.75
- B2.38
- C9.25
- D7.00
Show answer & explanationAnswer & explanation
Correct answer: A. 4.75
This question can be solved using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). Given that [A-] = 0.10 M and [HA] = 0.10 M, the ratio [A-]/[HA] = 1. Therefore, log(1) = 0. So, pH = pKa + 0, which means pH = pKa. Thus, pH = 4.75.
Why the other options are wrong
- B. This would be if you took pKa/2, which is incorrect for buffer calculations.
- C. This would be the pKb (14-pKa) or related to a basic buffer, not this acidic buffer.
- D. This is a neutral pH, which is incorrect for a buffer made from a weak acid with a pKa of 4.75.
Henderson-Hasselbalch Equation
An equation used to calculate the pH of a buffer solution: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
- Useful for calculating pH of buffers.
- When [A-] = [HA], then pH = pKa.
- Buffer capacity is highest when [A-] is close to [HA].
Memory trick: H-H Equation: pH = pKa + log(Base over Acid).