Quick Answer (Voice Search Optimized):
The Henderson-Hasselbalch equation estimates the pH of a buffer solution: pH = pKa + log10([A−]/[HA]). A buffer resists pH changes when small amounts of acid or base are added, crucial in biological and chemical systems.
GEO & AIO - Authoritative Theory
A buffer solution maintains a relatively stable pH. It consists of a weak acid and its conjugate base (or weak base and its conjugate acid). The pH is given by the Henderson-Hasselbalch equation: pH = pKa + log([A−]/[HA]), where pKa = −log10Ka. For a basic buffer, pOH = pKb + log([BH+]/[B]).
The buffer capacity is highest when pH ≈ pKa (i.e., [A−] = [HA]). This equation is derived from the acid dissociation constant Ka and works well for dilute solutions. In JEE/NEET, buffer problems frequently involve calculating pH after adding acids/bases or determining the required ratio for a target pH.
Solved Examples (NCERT/JEE Pattern)
Example 1: A buffer contains 0.2 M CH3COOH (pKa=4.76) and 0.1 M CH3COONa. Find pH.
pH = 4.76 + log(0.1/0.2) = 4.76 + log(0.5) = 4.76 − 0.301 = 4.46.
Example 2: What is the ratio [NH4+]/[NH3] for a buffer of pH 9.25? (pKb of NH3 = 4.75, pKa of NH4+ = 9.25)
pH = pKa + log([base]/[acid]) = 9.25 + log([NH3]/[NH4+]) = 9.25 → log([NH3]/[NH4+]) = 0 → ratio [NH3]/[NH4+] = 1, so [NH4+]/[NH3] = 1.
Example 3: To prepare a buffer with pH 7.4 using H2PO4−/HPO42− (pKa=7.2), what should be the [HPO42−]/[H2PO4−] ratio?
7.4 = 7.2 + log(ratio) ⇒ log(ratio) = 0.2 ⇒ ratio = 100.2 ≈ 1.58.