← Chemistry — Std 12
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Ionic Equilibria

Ch. 3Std 12

Easy Overview

Ever used a pH strip in biology class? That's ionic equilibria. Some compounds fall apart completely in water, some barely split. This chapter is about that tug-of-war between staying together and breaking apart.

Strong vs weak electrolytes

Strong electrolytes break apart completely in water — like NaCl. They don't hold back. Weak ones like acetic acid only partially break apart. It's like a party where some people dance (ions) and most just stand against the wall (undissociated molecules).

Ionization constant (Ka, Kb)

Every weak acid or base has a number that tells you how much it dissociates. Larger Ka = stronger acid (more willing to give up H+). It's like a see-saw — if the acid really wants to give away H+, the see-saw tips toward products.

pH and pOH

pH = -log[H+]. It's just a shortcut to say 'how acidic' without writing tiny numbers. 7 is neutral. Below is acidic. Above is basic. Each step of 1 means 10× more acidic — so pH 3 is 10× stronger than pH 4. Your stomach acid is around pH 2. Scary stuff.

Buffer solutions

A buffer resists pH changes. Like a shock absorber. Your blood is a buffer — it stays around pH 7.4 no matter what you eat. Buffers are usually a weak acid + its salt or weak base + its salt. Henderson-Hasselbalch equation is your best friend here.

Solubility product (Ksp)

Some compounds just won't dissolve much. Ksp tells you how much can dissolve before the solution says 'no more'. Like a crowded elevator — once it's full, no one else gets in. If the ion product exceeds Ksp, precipitation happens.

Key Points

  • •Strong electrolytes dissociate completely, weak ones partially
  • •Ka × Kb = Kw = 10^-14 at 25°C
  • •pH = -log[H+]; pOH = -log[OH-]; pH + pOH = 14
  • •Buffer resists pH change; blood buffer = H2CO3 / HCO3-
  • •Henderson-Hasselbalch: pH = pKa + log([salt]/[acid])
  • •Ksp = product of ion concentrations at saturation
  • •Common ion effect — adding a common ion suppresses dissociation

Practice Questions

  • Calculate the pH of 0.001 M HCl solution.
  • Explain how the common ion effect affects the solubility of AgCl in NaCl solution.
  • What is a buffer? Derive the Henderson-Hasselbalch equation for an acidic buffer.
  • The solubility of BaSO4 is 1.05 × 10^-5 M. Calculate Ksp.