CHEMISTRY • CLASS ALL • PHYSICAL CHEMISTRY PROBLEM SOLVING

Physical Chemistry: Mastering Mole, Equilibrium & Kinetics Numericals

Learn step-by-step problem-solving for Physical Chemistry in NEET. Master mole concept, buffer pH, solubility product (Ksp), and Arrhenius rate laws.

By NEETStudy Editorial Team • Reading Time: 11 min read • Last Updated: 2026-03-20
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♦ High-Yield Conceptual Takeaways

  • In equilibrium calculations, always verify whether the degree of dissociation alpha is small enough (< 0.05) before dropping it from (1 - alpha).
  • pH of dilute acid solutions (< 10^-6 M) must account for the autoionization of water ([H+]_w = 10^-7 M).
  • For first-order kinetics, t_1/2 is independent of concentration; use (1/2)^n fractional decay for speedy calculation.
  • In electrochemistry, remember: E°_cell is intensive (does not scale with stoichiometry), but Delta G° is extensive.

Physical Chemistry accounts for approximately one-third of the NEET Chemistry paper. Unlike Physics, where questions can present complex multi-body configurations, Physical Chemistry numericals follow predictable, well-defined mathematical pathways. Master the underlying algebra and logarithmic shortcuts to secure full marks in this section.

1. The Logarithmic Power Pack for NEET

Calculators are prohibited in NEET. Fast log calculation is an essential competitive advantage. Memorise these core anchors:

Base Anchor Value Derived Example Calculated Value
log 2 0.3010 log 4 = 2 * log 2 0.6020
log 3 0.4771 log 6 = log 2 + log 3 0.7781
log 5 0.6990 log 8 = 3 * log 2 0.9030
log 7 0.8450 log 9 = 2 * log 3 0.9542

2. Chemical & Ionic Equilibrium Mastery

Worked Example: Solubility Product (Ksp) with Common Ion Effect

Problem: Calculate the molar solubility of AgCl in a 0.1 M aqueous NaCl solution, given that the solubility product K_sp(AgCl) = 1.8 * 10^-10 at 298 K.

Step-by-Step Solution:

  1. NaCl is a strong electrolyte: NaCl → Na^+ (0.1 M) + Cl^- (0.1 M).
  2. AgCl is sparingly soluble: AgCl(s) ⇌ Ag^+(aq) + Cl^-(aq). Let solubility in this solution be s mol/L.
  3. At equilibrium: [Ag^+] = s and [Cl^-] = (s + 0.1) M.
  4. Solubility product expression: K_sp = [Ag^+] * [Cl^-] = s * (s + 0.1) = 1.8 * 10^-10.
  5. Since K_sp is very small, s ≪ 0.1, so (s + 0.1) ≈ 0.1.
  6. Therefore: s * 0.1 = 1.8 * 10^-10 ⇒ s = 1.8 * 10^-9 mol/L.

Conclusion: The solubility of AgCl in pure water is sqrt(1.8 * 10^-10) = 1.34 * 10^-5 M. In 0.1 M NaCl, common ion effect reduces its solubility by nearly four orders of magnitude!

3. Electrochemistry: The Nernst Equation Trap

At 298 K, the Nernst equation for a cell reaction is written as:

E_cell = E°_cell - (0.0591 / n) * log_10(Q)

Where n is the number of moles of electrons transferred in the balanced overall reaction, and Q is the reaction quotient (concentrations of aqueous products raised to stoichiometric coefficients divided by reactants; pure solids and liquids are taken as 1).

Common NEET Exam Trap:

Consider the Daniell cell: Zn(s) + Cu^2+(aq) → Zn^2+(aq) + Cu(s). Here n = 2 and Q = [Zn^2+] / [Cu^2+].

If the question says "Zn^2+ concentration is increased by 10 times", then log_10(10 * Q) = log_10(Q) + 1. The cell potential decreases by 0.0591 / 2 = 0.0295 V, not 0.0591 V!

Frequently Asked Student Questions

Should I memorize logarithm values for Physical Chemistry?

Yes! Memorise four basic log values: log 2 = 0.301, log 3 = 0.477, log 5 = 0.699, log 7 = 0.845. With these four, you can calculate the logarithm of almost any integer using log(a*b) = log a + log b.

What is the most common mistake in buffer solution pH calculations?

Confusing Henderson-Hasselbalch equations: Acidic buffer pH = pKa + log([Conjugate Base] / [Weak Acid]); Basic buffer pOH = pKb + log([Conjugate Acid] / [Weak Base]). For basic buffers, students frequently forget to subtract pOH from 14 to find pH.