Salt Hydrolysis
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Direct answer
Dissolve ammonium chloride in water and the solution turns acidic; dissolve sodium acetate and it turns basic. Both are salt hydrolysis — reaction of the salt's ions with water whenever one of them is the conjugate partner of a weak species. For a weak acid plus strong base salt (CH3COONa), pH = 7 + 1/2(pKa + log C); for a strong acid plus weak base salt (NH4Cl), pH = 7 − 1/2(pKb + log C); for a weak acid plus weak base salt, pH = 7 + 1/2(pKa − pKb) and is independent of concentration. Strong acid plus strong base salts (NaCl) do not hydrolyse at all, and their solutions stay at pH 7.
What you must remember
- Three formulas, one memory hook: acetate-type salts sit above 7, ammonium-type below 7, weak-weak salts wherever the pKa − pKb difference sends them.
- Hydrolysis constant: Kh = Kw/Ka for weak-acid salts, Kh = Kw/Kb for weak-base salts; smaller Ka means stronger hydrolysis and a more extreme pH.
- Degree of hydrolysis: h = sqrt(Kh/C), so hydrolysis deepens on dilution — pH of acetate or ammonium solutions drifts toward 7 as you dilute.
- Ammonium acetate neutrality: pKa of acetic acid equals pKb of ammonia at about 4.74, so pH = 7 + 1/2(4.74 − 4.74) = 7 exactly — a celebrated one-line result.
- Cation hydrolysis of high-charge metals: AlCl3 and FeCl3 solutions are acidic because hydrated Al^3+ and Fe^3+ ions behave as weak acids; Na2CO3 is basic because carbonate grabs protons.
- Buffer connection: a weak acid plus weak base salt solution carries both a weak acid and a weak base — the same equilibrium logic reappears in buffer questions.
Working the two mirror problems
Take 0.1 M NH4Cl with pKb of ammonia 4.74. Formula route: pH = 7 − 1/2(pKb + log C) = 7 − 1/2(4.74 + log 0.1) = 7 − 1/2(4.74 − 1) = 7 − 1.87 = 5.13. Now take 0.1 M CH3COONa with pKa of acetic acid 4.74: pH = 7 + 1/2(4.74 − 1) = 8.87. The two answers mirror each other about 7 because the pK values coincide — a symmetry worth seeing once so you never misplace a sign again.
Cross-check the acetate answer from first principles: [OH-] = sqrt(Kw × C/Ka) = sqrt(10^-14 × 0.1/1.8 × 10^-5) = sqrt(5.6 × 10^-11) = 7.5 × 10^-6 M, pOH = 5.13, pH = 8.87. The formula and the derivation agree, which is exactly the confidence you want before an integer-answer question where 8.87 and 5.13 are both options.
How the exam frames it
JEE Main tests the classification before the arithmetic: given the salt, decide the pH region, then compute. The high-frequency traps are the concentration term — every formula except the weak-weak one carries log C, so diluting an ammonium chloride solution pushes pH up toward 7, an assertion-reason favourite. The weak-weak independence is the opposite trap: the pH of ammonium acetate does not change at all on dilution, and students wrongly shift it. JEE Advanced enjoys justifying acidity from structure: sodium carbonate basic (carbonate hydrolyses), ferric chloride acidic (highly charged cation polarises O-H bonds of coordinated water), and sodium chloride neutral. Whenever an option list mixes AlCl3, Na2CO3, NH4Cl and CH3COONa, the reasoning chain — which ion hydrolyses, in which direction — answers all four at once.
Frequently asked questions
Why is an ammonium acetate solution neutral?
Because pH = 7 + 1/2(pKa − pKb), and for this salt the acid's pKa and the base's pKb are both 4.74, cancelling the correction term to zero.
What is the hydrolysis constant for sodium acetate?
Kh = Kw/Ka, the ionic product of water divided by the dissociation constant of acetic acid — the weaker the parent acid, the stronger the hydrolysis.
How does dilution affect the pH of a hydrolysing salt?
Degree of hydrolysis h = sqrt(Kh/C) rises on dilution, so acidic salts become less acidic and basic salts less basic — pH moves toward 7.
Why is an FeCl3 solution acidic even though FeCl3 looks like a strong acid-strong base salt?
Iron(III) hydroxide is a weak base, so the hydrated Fe^3+ cation releases protons to water; it is the cation that hydrolyses, not the chloride.
Which salt solutions are exactly neutral at 25°C?
Only salts of strong acids with strong bases (NaCl, KNO3), plus the special case of a weak-weak salt whose pKa equals pKb, such as ammonium acetate.