Ionic Equilibrium
Ionic Equilibrium tests acid–base strength, pH, buffers, salt hydrolysis, solubility product and indicators. Buffer and Ksp numericals are the main scoring areas.
Core rules
- pH = -log[H3O+]; pOH = -log[OH-]; pH + pOH = 14 at 25 °C.
- Weak acid: Ka = [H+][A-]/[HA]; for a weak acid, [H+] = sqrt(KaC).
- Buffer: pH = pKa + log([salt]/[acid]).
- Salt hydrolysis: strong acid–strong base salt is neutral; weak acid–strong base is basic; strong acid–weak base is acidic.
- Ksp is the product of ion concentrations in a saturated solution.
Worked example
A buffer contains 0.10 M acetic acid and 0.10 M sodium acetate. Ka = 1.8 x 10-5. Find the pH. Reasoning: pKa = -log(1.8 x 10-5) = 4.74. The ratio [salt]/[acid] = 0.10/0.10 = 1, so log 1 = 0. pH = 4.74 + 0 = 4.74. The buffer is acidic, as expected for an acetic acid–acetate buffer.
Traps
- pH + pOH = 14 is valid only at 25 °C.
- Do not use the strong-acid formula for a weak acid without checking the approximation.
- A salt of a weak acid and strong base gives pH > 7, not < 7.
- Common ions suppress ionisation; they do not increase Ksp.