Revision/Recall
1. TITLE
Fast Recall: Salt Hydrolysis Formulas Table cheat sheet — Everything for NEET on one page
2. QUICK FACTS BOX
SALT HYDROLYSIS — QUICK FACTS
* Salts from Strong Acids and Strong Bases do not hydrolyse.
* SA/SB salt solutions remain neutral (pH = 7).
* WA/SB salts undergo anionic hydrolysis, producing OH⁻ ions.
* SA/WB salts undergo cationic hydrolysis, producing H⁺ ions.
* WA/WB salts undergo both cationic and anionic hydrolysis.
* WA/WB salt pH is independent of the solution's concentration.
* Cations of strong bases and anions of strong acids only hydrate.
3. FORMULA SHEET
FORMULAS:
→ $pH = 7$: Use when calculating pH of a Strong Acid / Strong Base (SA/SB) salt.
→ $pH > 7$: Use when evaluating a Weak Acid / Strong Base (WA/SB) salt.
→ $pH < 7$: Use when evaluating a Strong Acid / Weak Base (SA/WB) salt.
→ $pH = 7 + \frac{1}{2}(pK_a - pK_b)$: Use when calculating pH of a Weak Acid / Weak Base (WA/WB) salt.
→ $pK_w = pH + pOH = 14$: Use when converting between pH and pOH at 298 K.
4. MUST-REMEMBER VALUES/CONSTANTS
KEY VALUES:
* $K_w$ = $1.0 \times 10^{-14} M^2$ (at 298 K)
* Neutral pH = 7 (at 298 K)
* $pK_w$ = 14 (at 298 K)
5. COMPARISON SUMMARY
| Salt Type | Parent Acid & Base | Hydrolysis Type | Solution pH (298 K) | Example |
|---|---|---|---|---|
| SA / SB | Strong Acid + Strong Base | None (Hydration only) | $pH = 7$ (Neutral) | $NaCl$ |
| WA / SB | Weak Acid + Strong Base | Anionic | $pH > 7$ (Basic) | $CH_3COONa$ |
| SA / WB | Strong Acid + Weak Base | Cationic | $pH < 7$ (Acidic) | $NH_4Cl$ |
| WA / WB | Weak Acid + Weak Base | Cationic & Anionic | $pH = 7 + \frac{1}{2}(pK_a - pK_b)$ | $CH_3COONH_4$ |
6. COMMON MISTAKES — QUICK LIST
DON'T:
* Assume all salts create neutral solutions in water—only SA/SB salts do.
* Try to use concentration values to calculate the pH of a WA/WB salt solution; it is concentration-independent.
* Mix up the hydrolysis types (e.g., SA/WB undergoes cationic hydrolysis, not anionic).
DO:
* Identify the parent acid and base strengths before predicting the pH of any salt solution.
* Remember that the $pH$ equation for WA/WB salts will yield a pH > 7 if $pK_a > pK_b$, and a pH < 7 if $pK_b > pK_a$.
7. MNEMONICS & MEMORY TRICKS
REMEMBER:
* The "Strong" Side Wins: If the base is strong (WA/SB), the pH is basic (>7). If the acid is strong (SA/WB), the pH is acidic (<7).
* Anions = Alkaline: Anionic hydrolysis happens with Weak Acid salts and makes the solution Alkaline (basic).
* Cations = aCidic: Cationic hydrolysis happens with Weak Base salts and makes the solution aCidic.
8. HIGH-YIELD NEET POINTS
NEET LOVES TO ASK:
* The direct calculation of WA/WB salt pH (like ammonium acetate) using the formula $pH = 7 + \frac{1}{2}(pK_a - pK_b)$.
* Identifying whether an aqueous solution of a given salt (e.g., $CH_3COONa$ or $NH_4Cl$) will be acidic, basic, or neutral.
* Confirming which specific ion undergoes hydrolysis (e.g., recognizing that in $CH_3COONa$, only the acetate anion hydrolyses to form $OH^-$).
9. ONE MCQ — SELF-TEST
Q: A salt of a weak acid and a strong base will result in an aqueous solution with a pH:
(a) Equal to 7
(b) Less than 7
(c) Greater than 7
(d) Equal to $pK_a$
Answer: (c) Greater than 7. The anion of the weak acid hydrolyses with water to produce $OH^-$ ions, rendering the solution basic.
10. NCERT PAGE REFERENCES
Revise from:
* Page 193-194: Ionic Product of Water & The pH Scale
* Page 201-202: Hydrolysis of Salts and the pH of their Solutions
11. CTA
- Detailed explanation → Equilibrium Guide
- Quick test → [Revision Test: 6-Equilibrium]