MHT-CET Chemistry · Ionic Equilibria

Salt Hydrolysis

When a salt dissolves, the ion coming from the weaker parent (weak acid or weak base) reacts with water, so the solution turns acidic, basic or neutral depending on which parent was weak.

All Ionic Equilibria notes MHT-CET Chemistry strategy

Why this matters

This is one of the most repeated Ionic Equilibria subtopics: almost every year a paper asks 'which salt turns litmus red/blue', 'which forms an acidic/basic solution', or 'which is NOT hydrolysed'. Nearly all of the PYQs are pure classification — decide the strong/weak nature of the parent acid and base and read off the result. A handful go further to the highest-pH comparison and the weak-acid/weak-base Ka-vs-Kb case, and the numeric formulas (Kh, degree of hydrolysis, pH) round out the theory a computation question can be built on.

Concept 1 of 3

The four salt types and their solution pH

Intuition

Every salt is the product of an acid and a base. Sort each parent as strong or weak, and the solution's pH follows a fixed rule: only the ion from the WEAKER parent reacts with water and shifts the pH. If both parents are strong, nothing hydrolyses and the solution stays neutral.

Definition

Classify by the strength of the parent acid and base:

Strong acid + strong base (e.g. $\text{NaCl}$ , $\text{KNO}_3$ , $\text{Na}_2\text{SO}_4$ ): no hydrolysis, solution is neutral, $\text{pH} = 7$ .
Strong acid + weak base (e.g. $\text{NH}_4\text{Cl}$ , $\text{CuSO}_4$ , $\text{CuCl}_2$ ): the cation hydrolyses, solution is acidic, $\text{pH} < 7$ .
Weak acid + strong base (e.g. $\text{CH}_3\text{COONa}$ , $\text{Na}_2\text{CO}_3$ , $\text{KCN}$ ): the anion hydrolyses, solution is basic, $\text{pH} > 7$ .
Weak acid + weak base (e.g. $\text{CH}_3\text{COONH}_4$ , $\text{NH}_4\text{CN}$ , $\text{NH}_4\text{F}$ ): BOTH ions hydrolyse; the pH **depends on $K_a$ vs $K_b$ ** — acidic if $K_a > K_b$ , basic if $K_b > K_a$ , neutral if $K_a = K_b$ .

Salt type	Example salt	Ion that hydrolyses	Solution / pH
Strong acid + strong base	$\text{NaCl}$ , $\text{KNO}_3$	None	Neutral, $\text{pH} = 7$ These salts are NOT hydrolysed — both ions come from strong parents and do not react with water.
Strong acid + weak base	$\text{NH}_4\text{Cl}$ , $\text{CuSO}_4$	Cation	Acidic, $\text{pH} < 7$
Weak acid + strong base	$\text{CH}_3\text{COONa}$ , $\text{Na}_2\text{CO}_3$	Anion	Basic, $\text{pH} > 7$
Weak acid + weak base	$\text{CH}_3\text{COONH}_4$ , $\text{NH}_4\text{CN}$	Both ions	Depends on $K_a$ vs $K_b$ $\text{NH}_4\text{CN}$ is basic because HCN ( $K_a \approx 4\times10^{-10}$ ) is a much weaker acid than $\text{NH}_4\text{OH}$ ( $K_b \approx 1.8\times10^{-5}$ ) is a base, so $K_b > K_a$ .

The pH is set by the WEAKER parent: weak base → acidic, weak acid → basic, both strong → neutral.

Practice this conceptself-check · 5 quick reps

Try it yourself

Among

\text{Na}_2\text{CO}_3

\text{NH}_4\text{Cl}

\text{NaCl}

and

\text{CH}_3\text{COONH}_4

, which aqueous solution has the highest pH?

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

1.
A salt of a strong acid and a strong base gives a solution of what pH?
2.
A salt of a strong acid and a weak base gives which type of solution?
3.
A salt of a weak acid and a strong base gives which type of solution?
4.
For a weak-acid + weak-base salt, what decides whether it is acidic or basic?
5.
Give one salt whose aqueous solution is NOT hydrolysed.

From the bank · past-year question

Example 1Ionic EquilibriaEASY

Which among the following salts forms basic solution in water?

[Q51 · 23 April Shift I · 2025]

A strong-acid + strong-base salt does NOT hydrolyse

Salts like

\text{NaCl}

\text{KNO}_3

and

\text{Na}_2\text{SO}_4

come from a strong acid AND a strong base, so neither ion reacts with water. The solution stays neutral — there is no hydrolysis at all. In a 'which is NOT hydrolysed' question, this is the answer.

Match the salt to the RIGHT parents

\text{Na}_2\text{SO}_4

is a favourite distractor in 'salt of strong acid and weak base' questions: it is actually strong acid (

\text{H}_2\text{SO}_4

) + strong base (NaOH), so it is neutral, NOT acidic. Always write out both parents before classifying.

Drill 7 more on the four salt types and their solution ph

Concept 2 of 3

Which ion hydrolyses — classifying a given salt

Intuition

Litmus and pH questions all reduce to one skill: name the parent acid and base of the salt, spot the weak one, and let its conjugate ion react with water. The ion from the strong parent is a spectator; the ion from the weak parent is the one that shifts the pH.

Definition

The classify-a-salt routine:

Split the salt into its cation (from a base) and anion (from an acid).
The ion from a weak parent hydrolyses; the ion from a strong parent does not.
Cation of a weak base (e.g. $\text{NH}_4^{+}$ , $\text{Cu}^{2+}$ ) hydrolyses → releases $\text{H}^{+}$ → acidic, turns blue litmus red.
Anion of a weak acid (e.g. $\text{CH}_3\text{COO}^{-}$ , $\text{CN}^{-}$ , $\text{CO}_3^{2-}$ ) hydrolyses → releases $\text{OH}^{-}$ → basic, turns red litmus blue.
If both ions come from strong parents, neither hydrolyses → neutral, no litmus change.

Salt	Weak parent	Ion that hydrolyses	Litmus effect
$\text{CuCl}_2$	Weak base $\text{Cu(OH)}_2$	$\text{Cu}^{2+}$ (cation)	Acidic — blue litmus turns red
$\text{NH}_4\text{NO}_3$	Weak base $\text{NH}_4\text{OH}$	$\text{NH}_4^{+}$ (cation)	Acidic — blue litmus turns red
$\text{CH}_3\text{COONa}$	Weak acid $\text{CH}_3\text{COOH}$	$\text{CH}_3\text{COO}^{-}$ (anion)	Basic — red litmus turns blue
$\text{KCN}$	Weak acid HCN	$\text{CN}^{-}$ (anion)	Basic — red litmus turns blue
$\text{NaNO}_3$	None (both strong)	Neither ion	Neutral — no litmus change $\text{NaNO}_3$ , NaCl and KCl are neutral — they are classic 'no change' distractors in litmus questions.

Weak-base cation → acidic (blue→red); weak-acid anion → basic (red→blue).

Practice this conceptself-check · 5 quick reps

Try it yourself

Which salt turns red litmus blue in aqueous solution:

\text{Na}_2\text{SO}_4

\text{CH}_3\text{COONa}

\text{NH}_4\text{NO}_3

\text{CuCl}_2

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

1.
In $\text{NH}_4\text{Cl}$ , which ion hydrolyses and what is the effect?
2.
In $\text{CH}_3\text{COONa}$ , which ion hydrolyses?
3.
A salt turns blue litmus red. What kind of salt is it?
4.
Does $\text{NaNO}_3$ change litmus colour?
5.
Which ion in $\text{CuSO}_4$ is responsible for its acidity?

From the bank · past-year question

Example 2Ionic EquilibriaEASY

Which among the following salts turns blue litmus red in its aqueous solution?

[Q51 · 25 April Shift I · 2025]

Only the ion of the WEAKER partner hydrolyses

The spectator ion (from the strong parent) does nothing. In

\text{CH}_3\text{COONa}

the

\text{Na}^{+}

is inert; it is the acetate anion (from weak

\text{CH}_3\text{COOH}

) that grabs

\text{H}^{+}

from water and leaves

\text{OH}^{-}

behind. Never let the strong-parent ion drive the pH.

Weak-base cation → acidic, not basic

Students sometimes assume an ammonium or copper salt is basic 'because it came from a base'. The opposite is true: a cation from a weak base (

\text{NH}_4^{+}

\text{Cu}^{2+}

) hydrolyses to give

\text{H}^{+}

, so the solution is acidic and turns blue litmus red.

Drill 7 more on which ion hydrolyses — classifying a given salt

Concept 3 of 3

Hydrolysis constant, degree of hydrolysis and pH

Intuition

The classification tells you the DIRECTION of the pH shift; these formulas tell you the SIZE of it. The hydrolysis constant is built from the water constant divided by the weak parent's ionisation constant, the degree of hydrolysis is how much of the ion actually reacts, and the pH follows from a compact half-formula.

Definition

For a salt of concentration $c$ :

Hydrolysis constant for a weak-acid/strong-base salt: $K_h = \dfrac{K_w}{K_a}$ ; for a strong-acid/weak-base salt: $K_h = \dfrac{K_w}{K_b}$ .
Degree of hydrolysis: $h = \sqrt{\dfrac{K_h}{c}} = \sqrt{\dfrac{K_w}{K_a\,c}}$ — the fraction of the ion that has reacted with water.
pH of a weak-acid + strong-base salt (basic): $\text{pH} = 7 + \tfrac{1}{2}\left(pK_a + \log c\right)$ .
pH of a strong-acid + weak-base salt (acidic): $\text{pH} = 7 - \tfrac{1}{2}\left(pK_b + \log c\right)$ .
For a weak-acid + weak-base salt the pH is concentration-independent: $\text{pH} = 7 + \tfrac{1}{2}\left(pK_a - pK_b\right)$ .

Hydrolysis constant, degree and salt pH

K_h = \dfrac{K_w}{K_a}\qquad h = \sqrt{\dfrac{K_h}{c}}\qquad \text{pH} = 7 + \tfrac{1}{2}\left(pK_a + \log c\right)

K_hhydrolysis constant of the salt
K_wionic product of water ( $10^{-14}$ at 298 K)
K_aionisation constant of the weak parent acid
K_bionisation constant of the weak parent base
hdegree of hydrolysis (fraction hydrolysed)
cmolar concentration of the salt

Worked example

The dissociation constant of acetic acid is

K_a = 1.0\times10^{-5}

. Find the degree of hydrolysis of

0.1\,\text{M}

sodium acetate. (

K_w = 1.0\times10^{-14}

)

Sodium acetate is a weak-acid + strong-base salt, so $K_h = \dfrac{K_w}{K_a}$ .
$K_h = \dfrac{1.0\times10^{-14}}{1.0\times10^{-5}} = 1.0\times10^{-9}$ .
Degree of hydrolysis $h = \sqrt{\dfrac{K_h}{c}} = \sqrt{\dfrac{1.0\times10^{-9}}{0.1}} = \sqrt{1.0\times10^{-8}}$ .
$\sqrt{1.0\times10^{-8}} = 1.0\times10^{-4}$ .

Answer:

h = 1.0\times10^{-4}

(about

0.01\%

of the acetate is hydrolysed).

Practice this conceptself-check · 4 quick reps

Try it yourself

A weak-acid + strong-base salt of concentration

0.01\,\text{M}

is made from an acid with

pK_a = 5

. Find the pH of the solution.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Write the hydrolysis constant of a weak-acid + strong-base salt in terms of $K_w$ and $K_a$ .
2.
If $K_h = 10^{-8}$ and $c = 0.01\,\text{M}$ , find the degree of hydrolysis.
3.
For a weak-acid + strong-base salt, does pH rise or fall as concentration falls?
4.
A weak-acid + weak-base salt has $pK_a = pK_b$ . What is its pH?

From the bank · past-year question

Example 3Ionic EquilibriaMODERATE

Which of the following aqueous solutions of salts will have highest pH value?

[Q65 · Shift 1 · 2022]

Divide $K_w$ by the WEAK parent's constant

For a weak-acid salt use

K_h = K_w/K_a

; for a weak-base salt use

K_h = K_w/K_b

. Picking the wrong constant (or using the salt's own 'K') gives a wrong

K_h

and hence a wrong

h

. A smaller

K_a

(weaker acid) means a LARGER

K_h

and more hydrolysis.

The degree of hydrolysis carries a square root

h = \sqrt{K_h/c}

, not

K_h/c

. If

K_h/c = 10^{-8}

, then

h = 10^{-4}

, not

10^{-8}

. Take the square root at the end, exactly as in weak-acid degree-of-dissociation problems.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Hydrolysis constant, degree of hydrolysis and pH
Hydrolysis constant, degree and salt pH
$K_h = \dfrac{K_w}{K_a}\qquad h = \sqrt{\dfrac{K_h}{c}}\qquad \text{pH} = 7 + \tfrac{1}{2}\left(pK_a + \log c\right)$

Reference tables (2)

The four salt types and their solution pH4 rows

Salt type	Example salt	Ion that hydrolyses	Solution / pH
Strong acid + strong base	$\text{NaCl}$ , $\text{KNO}_3$	None	Neutral, $\text{pH} = 7$ These salts are NOT hydrolysed — both ions come from strong parents and do not react with water.
Strong acid + weak base	$\text{NH}_4\text{Cl}$ , $\text{CuSO}_4$	Cation	Acidic, $\text{pH} < 7$
Weak acid + strong base	$\text{CH}_3\text{COONa}$ , $\text{Na}_2\text{CO}_3$	Anion	Basic, $\text{pH} > 7$
Weak acid + weak base	$\text{CH}_3\text{COONH}_4$ , $\text{NH}_4\text{CN}$	Both ions	Depends on $K_a$ vs $K_b$ $\text{NH}_4\text{CN}$ is basic because HCN ( $K_a \approx 4\times10^{-10}$ ) is a much weaker acid than $\text{NH}_4\text{OH}$ ( $K_b \approx 1.8\times10^{-5}$ ) is a base, so $K_b > K_a$ .

The pH is set by the WEAKER parent: weak base → acidic, weak acid → basic, both strong → neutral.

Which ion hydrolyses — classifying a given salt5 rows

Salt	Weak parent	Ion that hydrolyses	Litmus effect
$\text{CuCl}_2$	Weak base $\text{Cu(OH)}_2$	$\text{Cu}^{2+}$ (cation)	Acidic — blue litmus turns red
$\text{NH}_4\text{NO}_3$	Weak base $\text{NH}_4\text{OH}$	$\text{NH}_4^{+}$ (cation)	Acidic — blue litmus turns red
$\text{CH}_3\text{COONa}$	Weak acid $\text{CH}_3\text{COOH}$	$\text{CH}_3\text{COO}^{-}$ (anion)	Basic — red litmus turns blue
$\text{KCN}$	Weak acid HCN	$\text{CN}^{-}$ (anion)	Basic — red litmus turns blue
$\text{NaNO}_3$	None (both strong)	Neither ion	Neutral — no litmus change $\text{NaNO}_3$ , NaCl and KCl are neutral — they are classic 'no change' distractors in litmus questions.

Weak-base cation → acidic (blue→red); weak-acid anion → basic (red→blue).

Watch out for (6)

A strong-acid + strong-base salt does NOT hydrolyse
→ The four salt types and their solution pH
Match the salt to the RIGHT parents
→ The four salt types and their solution pH
Only the ion of the WEAKER partner hydrolyses
→ Which ion hydrolyses — classifying a given salt
Weak-base cation → acidic, not basic
→ Which ion hydrolyses — classifying a given salt
Divide $K_w$ by the WEAK parent's constant
→ Hydrolysis constant, degree of hydrolysis and pH
The degree of hydrolysis carries a square root
→ Hydrolysis constant, degree of hydrolysis and pH

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Ionic EquilibriaEASY

Which among the following is NOT an example of salt of strong acid and weak base?

[Q81 · 12th May Shift 2 · 2024]

Example 2Ionic EquilibriaMODERATE

Which of the following salts turns red litmus blue in its aqueous solution?

[Q63 · 11th May Shift 1 · 2024]

Example 3Ionic EquilibriaEASY

Which among the following salts is NOT hydrolyzed in water?

[Q99 · 19 April Shift I · 2025]

Example 4Ionic EquilibriaEASY

Which among the following salts turns red litmus blue in its aqueous solution?

[Q62 · 26 April Shift II · 2025]

Example 5Ionic EquilibriaEASY

Which salt from following forms aqueous solution having pH less than 7?

[Q89 · 9th May Shift 1 · 2024]

Drill every past-year question on this subtopic

17 questions from the bank — paginated, with cart and Word-export support.

Drill the 17 questions

Related notes

Salts and Common Compounds (NDA Chemistry) →