Hydrolysis of Salts & pH

Salts can interact with water (hydrolysis!), changing the solution’s pH:

  • 👉 Weak acid + Strong base salt (e.g., CH3COONa):
    CH3COO– + H2O ⇌ CH3COOH + OH–
    → pH > 7 (Alkaline)
  • 👉 Strong acid + Weak base salt (e.g., NH4Cl):
    NH4+ + H2O ⇌ NH4OH + H+
    → pH < 7 (Acidic)
  • 👉 Weak acid + Weak base salt (e.g., CH3COONH4):
    Both ions hydrolyze! pH depends on \( K_a \) and \( K_b \): \[ \text{pH} = 7 + \frac{1}{2} (\text{p}K_a – \text{p}K_b) \] Example: For ammonium acetate (pKa = 4.76, pKb = 4.75): \[ \text{pH} = 7 + \frac{1}{2}(4.76 – 4.75) = 7.005 \approx 7 \text{ (Neutralish!)} \]

Buffer Solutions 🧪

Definition: Solutions that resist pH changes when diluted or when small amounts of acid/base are added. Super important for blood, medicines, and labs!

  • 👉 Acidic Buffer: Weak acid + Its salt with strong base (e.g., CH3COOH + CH3COONa).
  • 👉 Basic Buffer: Weak base + Its salt with strong acid (e.g., NH4OH + NH4Cl).

Henderson-Hasselbalch Equation (for pH calculation):

  • For Acidic Buffer: \[ \text{pH} = \text{p}K_a + \log\frac{[\text{Salt}]}{[\text{Acid}]} \] Pro Tip 🎯: When [Salt] = [Acid], pH = pKa! (e.g., pH = 4.76 for equal CH3COOH/CH3COONa).
  • For Basic Buffer: \[ \text{pOH} = \text{p}K_b + \log\frac{[\text{Conjugate Acid}]}{[\text{Base}]} \] → Then use: pH = 14 – pOH

✨ Cool fact: Diluting a buffer doesn’t change its pH! (The log ratio stays the same).

Solubility Equilibria ⚖️

Some salts are “sparingly soluble” (e.g., BaSO4, LiF). Their saturation equilibrium is:

Example for BaSO4: \[ \text{BaSO}_{4(s)} \rightleftharpoons \text{Ba}^{2+}_{(aq)} + \text{SO}_4^{2-}_{(aq)} \]

Solubility Product Constant (Ksp): \[ K_{sp} = [\text{Ba}^{2+}][\text{SO}_4^{2-}] = 1.1 \times 10^{-10} \ \text{(at 298 K)} \]

Molar Solubility (S): Moles of salt dissolving per liter. For BaSO4: \[ K_{sp} = S \times S = S^2 \rightarrow S = \sqrt{1.1 \times 10^{-10}} = 1.05 \times 10^{-5}\ \text{mol/L} \]

For salts like Zr3(PO4)4 (which gives 3 Zr4+ and 4 PO43- ions): \[ K_{sp} = (3S)^3 \times (4S)^4 = 6912 \ S^7 \]

🌟 High-Yield NEET Concepts:

  1. Salt Hydrolysis & pH Prediction: Know how weak/strong acid-base pairs affect pH (NEET loves classification!).
  2. pH of Weak Acid-Weak Base Salts: Master \( \text{pH} = 7 + \frac{1}{2}(\text{p}K_a – \text{p}K_b) \).
  3. Buffer pH Calculations: Henderson-Hasselbalch equation is must-know for acidic/buffers.
  4. Ksp vs. Solubility: Relate \( K_{sp} \) to molar solubility (S) for salts (even with different ions!).
  5. Buffer Action: Why buffers resist pH change (dilution/additives) is frequently tested.

Keep practicing hydrolysis equations and buffer problems – you’ve got this! 💪