pH ScaleJEE Main

Concept

pH measures how acidic or basic a solution is on a log scale: $\mathrm{pH} = -\log_{10}[H^+]$. A unit drop in pH means ten times more $H^+$. At 25 °C, $\mathrm{pH}+\mathrm{pOH}=14$, with pH 7 neutral, below 7 acidic, above 7 basic.

Key formula

Derivation

A strong acid dissociates fully, so $[H^+]=C$ and $\mathrm{pH}=-\log C$.

A weak acid only partly dissociates: $[H^+]=\sqrt{K_a C}$, giving $\mathrm{pH}=\tfrac12(\mathrm{p}K_a-\log C)$. Bases mirror this through pOH, and $\mathrm{pH}=14-\mathrm{pOH}$.

Scenarios to explore

• pH Scale — pH and pOH of strong & weak acids and bases.

Real-world applications

• Buffer and titration calculations.
• Biological systems (blood pH ≈ 7.4).
• Indicator colour changes near the equivalence point.

JEE exam tips

• Strong acid: pH = −log C directly.
• Weak acid: pH = ½(pKa − log C).
• Diluting a strong acid 10× raises pH by exactly 1 (until near 7).

Common mistakes

• Treating a weak acid as fully ionized (overestimating [H⁺]).
• Forgetting pH + pOH = 14 only holds at 25 °C.
• Reporting pH outside 0–14 for ordinary dilute solutions.

Exam traps to avoid

• Very dilute strong acid (≤10⁻⁷ M) needs water's autoionization — pH never crosses 7.
• pKa is for the acid; for its conjugate base use pKb = 14 − pKa.