JEE Mains Chemistry · Equilibrium

Salt Hydrolysis and Indicators

Finding the pH of a salt solution from the strengths of the acid and base that formed it, and choosing an acid-base indicator by its colour-change range.

All Equilibrium notes JEE Mains Chemistry strategy

Why this matters

Fourteen PYQs, ten of them multiple choice, and none yet from 2026. Five find the pH of a salt solution; nine test indicators: which one suits which titration, and what form it takes in acid and in base.

Concept 1 of 2: pH of a salt solution

A salt's ion reacts with water only if it came from a weak partner. Acetate, from a weak acid, takes

\mathrm{H^+}

from water and leaves

\mathrm{OH^-}

, so sodium acetate is basic. Ammonium, from a weak base, gives

\mathrm{H^+}

to water, so ammonium chloride is acidic. When both ions hydrolyse, the stronger partner wins.

Definition

Weak acid + strong base salt ( $\mathrm{CH_3COONa}$ ): basic, $\mathrm{pH}=7+\tfrac12\mathrm{p}K_a+\tfrac12\log C$ , C = concentration of the anion.
Strong acid + weak base salt ( $\mathrm{NH_4Cl}$ ): acidic, $\mathrm{pH}=7-\tfrac12\mathrm{p}K_b-\tfrac12\log C$ .
Weak acid + weak base salt ( $\mathrm{CH_3COONH_4}$ ): $\mathrm{pH}=7+\tfrac12(\mathrm{p}K_a-\mathrm{p}K_b)$ , independent of C.
Strong acid + strong base salt (NaCl): no hydrolysis, pH 7.
For an anion such as $\mathrm{CO_3^{2-}}$ , use the Ka of its own conjugate acid, $\mathrm{HCO_3^-}$ ( $K_{a2}$ of carbonic acid). Its $K_h=K_w/K_{a2}$ is far larger than that of $\mathrm{NH_4^+}$ , so ammonium carbonate is basic.

Salt of a weak acid and a strong base

\mathrm{pH}=7+\tfrac12\,\mathrm{p}K_a+\tfrac12\log C

Worked example

Find the pH of 0.1 M sodium acetate.

\mathrm{p}K_a(\mathrm{CH_3COOH})=4.74

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 25 July 2022 · Q34Moderate

Example 1 · Equilibrium · Salt Hydrolysis and Indicators

20\text{ }mL

0.1MNH_{4}OH

is mixed with

40\text{ }mL

0.05MHCl

. The

pH

of the mixture is nearest to: (Given:

K_{b}\left( NH_{4}OH \right)= 1 \times10^{- 5},\log2 = 0.30

\log3 = 0.48,\log5 = 0.69,\log7 = 0.84

\log11 = 1.04)

Use the anion's concentration

Calcium lactate gives two lactate ions per formula, so a 0.005 M solution has 0.01 M lactate. Put 0.01 into the formula, not 0.005.

Exact neutralisation leaves a salt, not a buffer

Equal moles of a weak base and HCl leave only the salt, in the total volume. Use the hydrolysis formula, not the Henderson equation, and divide by the combined volume.

Drill 4 more on ph of a salt solution

Concept 2 of 2: Acid-base indicators and titrations

An indicator is itself a weak acid or base whose two forms have different colours. It changes colour over about two pH units around its own pK. It works only if that range falls on the steep jump of the titration curve, which is why the right indicator depends on which partners are strong.

Definition

An indicator HIn changes colour over roughly $\mathrm{p}K_{In}\pm1$ .
Phenolphthalein: $K_{In}=4\times10^{-10}$ gives $\mathrm{p}K_{In}\approx9.4$ , so it starts to change near 8.4.
Weak acid vs strong base: the equivalence point is above 7, so phenolphthalein.
Strong acid vs weak base: the equivalence point is below 7, so methyl orange.
Strong acid vs strong base: the jump is long, so either works.
Weak acid vs weak base: no sharp jump, so no indicator gives a clear end point.
A titration curve for acid added to a weak base falls slowly through a buffer region, then drops to an acidic equivalence point.

Indicator	Nature	Colour in acid, then in base	Works around	Used for
Phenolphthalein	Weak acid	Colourless (un-ionised HIn), then pink (ionised $\mathrm{In^-}$ )	pH 8.3 to 10	Weak acid vs strong base; strong acid vs strong base It does ionise in base; the pink colour is the ionised form.
Methyl orange	Weak base	Red quinonoid form, then yellow benzenoid form	pH 3.1 to 4.4	Strong acid vs weak base; strong acid vs strong base The quinonoid (acid) form is the more deeply coloured one.
Eriochrome black T	Metal-ion indicator	Wine red with $\mathrm{Ca^{2+}}$ or $\mathrm{Mg^{2+}}$ , then blue when free	pH about 10	EDTA titrations of $\mathrm{Ca^{2+}}$ and $\mathrm{Mg^{2+}}$
Diphenylamine	Redox indicator	Colourless when reduced, then violet when oxidised	A change in electrode potential, not pH	Redox titrations, such as $\mathrm{Fe^{2+}}$ with dichromate

Acid-base indicators respond to pH; redox indicators respond to electrode potential.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 31 January 2023 · Q132Moderate

Example 2 · Equilibrium · Salt Hydrolysis and Indicators

Incorrect statement for the use of indicators in acid-base titration is:

Phenolphthalein is an acid, and it ionises in base

Phenolphthalein is colourless in acid and pink in base because its ionised form is pink. "It does not dissociate in basic medium" is false.

Redox and acid-base indicators are not swapped

Acid-base indicators respond to pH; redox indicators respond to electrode potential. A statement pair that swaps the two makes both statements false.

Drill 8 more on acid-base indicators and titrations

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)

pH of a salt solution
Salt of a weak acid and a strong base
$\mathrm{pH}=7+\tfrac12\,\mathrm{p}K_a+\tfrac12\log C$

Reference tables (1)

Acid-base indicators and titrations4 rows

Indicator	Nature	Colour in acid, then in base	Works around	Used for
Phenolphthalein	Weak acid	Colourless (un-ionised HIn), then pink (ionised $\mathrm{In^-}$ )	pH 8.3 to 10	Weak acid vs strong base; strong acid vs strong base It does ionise in base; the pink colour is the ionised form.
Methyl orange	Weak base	Red quinonoid form, then yellow benzenoid form	pH 3.1 to 4.4	Strong acid vs weak base; strong acid vs strong base The quinonoid (acid) form is the more deeply coloured one.
Eriochrome black T	Metal-ion indicator	Wine red with $\mathrm{Ca^{2+}}$ or $\mathrm{Mg^{2+}}$ , then blue when free	pH about 10	EDTA titrations of $\mathrm{Ca^{2+}}$ and $\mathrm{Mg^{2+}}$
Diphenylamine	Redox indicator	Colourless when reduced, then violet when oxidised	A change in electrode potential, not pH	Redox titrations, such as $\mathrm{Fe^{2+}}$ with dichromate

Acid-base indicators respond to pH; redox indicators respond to electrode potential.

Watch out for (4)

Use the anion's concentration
→ pH of a salt solution
Exact neutralisation leaves a salt, not a buffer
→ pH of a salt solution
Phenolphthalein is an acid, and it ionises in base
→ Acid-base indicators and titrations
Redox and acid-base indicators are not swapped
→ Acid-base indicators and titrations

Test yourself on Equilibrium

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