Ph Scale

The high-frequency trap in pH problems: treating a weak acid as if it fully dissociates. A 0.1 M acetic acid solution does NOT have pH = 1. Only a fraction ionises, and ignoring that costs you the question outright.

pH and pOH — the definitions. pH = −log₁₀[H⁺]. pOH = −log₁₀[OH⁻]. At 25 °C, pH + pOH = 14 because Kw = [H⁺][OH⁻] = 10⁻¹⁴ (NCERT Class 11 Chemistry Chapter 7, page 24). Pure water: [H⁺] = [OH⁻] = 10⁻⁷ mol/L, so pH = pOH = 7.

Strong vs weak — the fork that decides your method. For a strong acid like HCl at concentration C, assume complete dissociation: [H⁺] = C, pH = −log C. For a weak acid (acetic acid, Ka ≈ 1.8 × 10⁻⁵), only a small fraction α ionises. The working formula is [H⁺] ≈ √(Ka · C), valid when α is small (C ≫ Ka). This is where most NEET errors originate — students skip the √(Ka · C) step and plug C directly into pH = −log C.

The Ka–Kb–Kw bridge. For a conjugate acid-base pair, Ka × Kb = Kw. Equivalently, pKa + pKb = 14. If a question gives Kb for ammonia and asks pH of ammonium chloride solution, you need Ka = Kw/Kb first.

Logarithm arithmetic. NEET pH questions reduce to log manipulation. Know these cold: log 2 ≈ 0.301, log 3 ≈ 0.477, log 5 ≈ 0.699. A one-unit pH change is a tenfold change in [H⁺].

Watch-out: when the problem says "weak acid" but gives a neat concentration like 0.01 M, the temptation is to write pH = 2 directly. That only works for strong acids. For weak acids, always reach for [H⁺] ≈ √(Ka · C).

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