Buffer SolutionsJEE Main

Henderson–Hasselbalch pH — interactive Chemistry simulation for IIT-JEE.

Concept

A buffer resists pH change when small amounts of acid or base are added. It is a weak acid mixed with its conjugate base; the Henderson–Hasselbalch equation gives its pH from the simple ratio of the two.

Key formula

\text{pH} = \text{p}K_a + \log\frac{[A^-]}{[HA]}

Derivation

Starting from $K_a = \dfrac{[H^+][A^-]}{[HA]}$ , take $-\log$ of both sides: $\text{pH} = \text{p}K_a + \log\dfrac{[A^-]}{[HA]}$ .

When $[A^-] = [HA]$ the log term is zero, so $\text{pH} = \text{p}K_a$ — and the buffer is at maximum capacity, resisting change best within roughly ±1 pH of $\text{p}K_a$ .

Scenarios to explore

Buffer Solutions — Henderson–Hasselbalch pH and capacity.

Real-world applications

Blood pH held near 7.4 by the carbonic-acid/bicarbonate buffer.
Maintaining pH in fermentation, electroplating and biochemistry.
Calibration standards for pH meters.

JEE exam tips

Pick an acid whose $\text{p}K_a$ is within ±1 of the target pH for an effective buffer.
Equal concentrations ⇒ pH = p $K_a$ — the fastest mental check.

Common mistakes

Inverting the ratio — it is base over acid.
Using a buffer far from its $\text{p}K_a$ , where capacity is poor.
Forgetting added strong acid/base shifts the ratio before you re-apply the equation.

Exam traps to avoid

Diluting a buffer barely changes its pH (the ratio is unchanged).
Henderson–Hasselbalch assumes the weak-acid approximation holds (not at extreme ratios).