Question
A buffer solution is prepared by mixing 0.2 M acetic acid and 0.3 M sodium acetate in water. Find the pH of the buffer. Given of acetic acid . (Real-world: this is the pH range of vinegar-based buffers used in food preservation.)
Solution — Step by Step
Acetic acid (weak acid) + sodium acetate (its conjugate base salt) — classic acid buffer. Use the Henderson-Hasselbalch equation:
pH of the buffer ≈ 4.92.
Why This Works
A buffer resists pH changes because it has both an acid (to neutralize added base) and a conjugate base (to neutralize added acid). The Henderson-Hasselbalch equation comes from rearranging the expression assuming the salt and acid concentrations don’t change much during the buffering action.
For maximum buffering capacity, the salt:acid ratio should be 1:1, giving pH = p. Our buffer at 1.5:1 is still effective and within the useful range (typically p).
For basic buffers (weak base + its salt), use , then pH = 14 - pOH. Don’t directly apply the acid version.
Alternative Method
From first principles: write the equilibrium expression . Salt fully dissociates, so M. Acid is weak, so M (negligible dissociation suppressed by the salt — common ion effect). Solving: . pH . Same answer.
Common Mistake
Students forget that the Henderson-Hasselbalch equation uses log to base 10, not natural log. Also, for a basic buffer, applying the acid version directly gives the wrong answer — use pOH first.