Solubility Product

8 MCQs2 revision cards9-step worked example

Source: NCERT EquilibriumPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The trap that costs marks in Ksp problems is almost always the same: forgetting stoichiometric powers. For a salt like CaF₂, students write Ksp = [Ca²⁺][F⁻] instead of Ksp = [Ca²⁺][F⁻]². That single missing exponent turns every downstream calculation wrong and hands away 4 marks under negative marking.

What Ksp actually is. When a sparingly soluble salt MₐXᵦ is placed in water, a tiny amount dissolves and establishes an equilibrium:

MₐXᵦ(s) ⇌ aM⁺(aq) + bX⁻(aq)

The solubility product is the equilibrium constant for this process (NCERT Class 11 Chemistry Chapter 7, page 38):

Ksp = [M⁺]ᵃ · [X⁻]ᵇ

The solid does not appear — its activity is 1 by convention.

Connecting solubility (s) to Ksp. If the molar solubility is s, then [M⁺] = as and [X⁻] = bs. For a 1:1 salt like AgCl: Ksp = s². For a 1:2 salt like CaF₂: Ksp = (s)(2s)² = 4s³. For a 2:1 salt like Ag₂CrO₄: Ksp = (2s)²(s) = 4s³. The stoichiometry changes the algebraic form entirely.

Predicting precipitation. Compare the ionic product Q with Ksp:

Q < Ksp → unsaturated, more salt dissolves.
Q = Ksp → saturated, equilibrium.
Q > Ksp → supersaturated, precipitation occurs.

Common-ion effect. Adding a common ion (say, F⁻ from NaF to a saturated CaF₂ solution) increases [F⁻], pushing Q above Ksp. The system responds by precipitating CaF₂ until Q falls back to Ksp. This lowers the effective solubility of the salt.

Watch out: when a problem gives you a non-1:1 salt and asks for solubility, write the dissolution equation first, assign stoichiometric coefficients to s, then build the Ksp expression. Skipping this step is how the stoichiometric-power trap catches you.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The solubility product expression for Mg(OH)₂ is:

MCQ 2Direct ApplicationPractice

The Ksp of AgCl is 1.8 × 10⁻¹⁰. What is the molar solubility of AgCl in pure water?

MCQ 3Direct ApplicationPractice

For CaF₂ (Ksp = 3.2 × 10⁻¹¹), the molar solubility in pure water is closest to:

MCQ 4Easy RecallPractice

What is the SI unit of Ksp for a salt of type AB₂ (e.g., CaF₂)?

MCQ 5Easy RecallPractice

If the ionic product Q for PbCl₂ in a solution is 2.5 × 10⁻⁴ and Ksp of PbCl₂ is 1.7 × 10⁻⁵, what will happen?

MCQ 6Direct ApplicationPractice

The solubility of Ag₂CrO₄ in pure water is s mol/L. Its Ksp expression in terms of s is:

MCQ 7CalculationPractice

The Ksp of BaSO₄ is 1.1 × 10⁻¹⁰. If 0.01 M Na₂SO₄ is added to a saturated BaSO₄ solution, the new solubility of BaSO₄ is approximately:

MCQ 8CalculationPractice

For a sparingly soluble salt M₃(PO₄)₂ with molar solubility s, the Ksp expression is:

Quick recall before you leave

Worked Example

Pattern: NEET pattern: solubility ksp — compute solubility from Ksp for a salt with stoichiometric coefficients > 1.

1
Given
Ksp of Ca₃(PO₄)₂ = 2.07 × 10⁻³³. The salt is dissolved in pure water at 25°C.
2
Required
Find the molar solubility (s) of Ca₃(PO₄)₂ in pure water.
3
Concept
For a sparingly soluble salt, the dissolution equilibrium defines Ksp. The stoichiometric coefficients become powers in the Ksp expression (NCERT Class 11 Chemistry Chapter 7, page 38).
4
Formula
Ca₃(PO₄)₂ ⇌ 3Ca²⁺ + 2PO₄³⁻ Ksp = [Ca²⁺]³ · [PO₄³⁻]² If molar solubility = s: [Ca²⁺] = 3s, [PO₄³⁻] = 2s Ksp = (3s)³(2s)² = 27s³ · 4s² = 108s⁵
5
Substitution
2.07 × 10⁻³³ = 108s⁵ s⁵ = 2.07 × 10⁻³³ / 108 = 1.917 × 10⁻³⁵
6
Calculation
s = (1.917 × 10⁻³⁵)^(1/5) Take the fifth root: 1.917^(1/5) ≈ 1.139; (10⁻³⁵)^(1/5) = 10⁻⁷ s ≈ 1.14 × 10⁻⁷ mol/L Note on exact constants: the coefficient 108 is an exact integer derived from stoichiometry (3³ × 2² = 108). It does not limit significant figures. The answer precision is governed by the 3 significant figures in Ksp = 2.07 × 10⁻³³.
7
Final answer
The molar solubility of Ca₃(PO₄)₂ in pure water is approximately **1.14 × 10⁻⁷ mol/L**.
8
Common trap
Writing Ksp = s·s = s² (treating the salt as 1:1) or Ksp = 3s·2s = 6s² (using coefficients as multipliers without raising to powers). The correct form demands (3s)³ and (2s)², producing 108s⁵. This is the stoichiometric-power trap that appears frequently in NEET solubility-product questions.
9
Similar NEET-style question
The Ksp of Ag₂CrO₄ is 1.12 × 10⁻¹². Calculate its molar solubility in pure water. *(Hint: Ag₂CrO₄ ⇌ 2Ag⁺ + CrO₄²⁻, so Ksp = 4s³.)* ---

Before solving, remember these

Formula

Solubility product

For sparingly soluble salt M_aX_b ⇌ aM⁺ + bX⁻: K_sp = [M⁺]^a[X⁻]^b. Q < K_sp: more dissolves. Q > K_sp: precipitates. Q = K_sp: saturated.

-- NCERT Class 11 Chemistry, Ch. 6, p. 38

Formulas

Henderson-Hasselbalch (buffer)

pH = p K_{a} + lo g \frac{[ salt ]}{[ acid ]}

pH of acidic buffer in terms of conjugate base/acid concentrations. For basic buffer: pOH = pKb + log10([salt]/[base]).

Symbol	Quantity	SI Unit
pKa	-log Ka	-
[salt]	conjugate base conc	mol/L
[acid]	weak acid conc	mol/L

Valid when

Buffer (weak acid + conjugate base)
Concentrations not too dilute
Approximate (assumes negligible dissociation)

Ka, Kb, Kw relationship

K_{a} K_{b} = K_{w}

Stronger acid → weaker conjugate base, and vice versa. pKa + pKb = 14.

Symbol	Quantity	SI Unit
Ka	acid dissociation	-
Kb	base dissociation	-
Kw	water 10^-14	-

Valid when

Conjugate acid-base pair
25°C

Equilibrium constant K_p and K_c

K_{p} = K_{c} (R T)^{Δ n}

Convert between pressure-based and concentration-based equilibrium constants. T in K; R = 0.0821 L atm/mol/K (when K_p in atm).

Symbol	Quantity	SI Unit
K_p	pressure constant	-
K_c	concentration constant	-
Δn	mole change	-

Valid when

Gas-phase equilibrium
Same temperature

Solubility product

K_{sp} = [M^{+}]^{a} [X^{-}]^{b}

Equilibrium constant for sparingly soluble salt. Q < K_sp: dissolves; Q > K_sp: precipitates.

Symbol	Quantity	SI Unit
K_sp	solubility product	-

Valid when

Sparingly soluble salt
Saturated solution

pH and pOH

pH = - lo g [H^{+}]

Logarithmic acidity scale. Pure water at 25°C: pH = 7 = pOH.

Symbol	Quantity	SI Unit
[H+]	hydrogen ion conc	mol/L
[OH-]	hydroxide conc	mol/L

Valid when

Aqueous solution
Use Kw = 10^-14 at 25°C

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Catalyst does NOT shift equilibrium

Category: Overthinking

Student claims catalyst shifts equilibrium toward products. Catalyst speeds up forward AND reverse equally; equilibrium position unchanged.

When it triggers

Question lists catalyst addition among options for shifting equilibrium.

How to avoid

Catalyst lowers activation energy of BOTH forward and reverse reactions equally. Time to reach equilibrium decreases; equilibrium position is unchanged.

Trap

Δn sign in K_p ↔ K_c conversion

Category: Sign Convention

Δn = (mol gas product) - (mol gas reactant). Sign matters; K_p = K_c (RT)^Δn.

When it triggers

Convert K_p ↔ K_c for gas-phase reaction.

How to avoid

Count moles of gas only (ignore solids/liquids). Δn = product - reactant. If Δn = 0, K_p = K_c. If Δn = +1, K_p = K_c × RT.

Trap

K_sp stoichiometric powers

Category: Similar Terms

For salt M_aX_b: K_sp = [M⁺]^a [X⁻]^b. Student uses [M⁺][X⁻] regardless of stoichiometry.

When it triggers

K_sp problem with non-1:1 salt (e.g. CaF₂, Mg(OH)₂, Ag₂CrO₄).

How to avoid

Write dissolution: M_aX_b → aM + bX. Then K_sp = [M]^a · [X]^b. For CaF₂ ↔ Ca + 2F: K_sp = s · (2s)² = 4s³.

Mistake

Believes catalyst shifts equilibrium toward products.

Root cause: concept gap

Correction

Catalyst speeds up forward AND reverse equally. Equilibrium position is unchanged. Time to reach equilibrium decreases.

Mistake

Counts solid/liquid moles in Δn for K_p ↔ K_c conversion.

Root cause: concept gap

Correction

Δn = (moles GAS product) - (moles GAS reactant). Solids and liquids excluded.

Mistake

Uses K_sp = [M+][X-] regardless of salt stoichiometry.

Root cause: formula misuse

Correction

For M_aX_b: K_sp = [M]^a · [X]^b. CaF₂: K_sp = s · (2s)² = 4s³.

Mistake

Predicts wrong direction of shift on pressure change.

Root cause: concept gap

Correction

Increase P → shift toward FEWER moles of gas. Decrease P → shift toward MORE moles. Reactions with same number of moles of gas: P doesn't shift equilibrium.

Mistake

Treats weak acid as fully dissociated when computing pH.

Root cause: concept gap

Correction

Weak acid: only fraction ionises. [H+] ≈ √(Ka · C). For 0.1 M acetic acid (Ka ~10⁻⁵): [H+] ~10⁻³ (pH ~3), not 1 (pH=1).

Past Year Questions

15 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Among the following, choose the ones with equal number of atoms. A. 212 g of Na CO (s) [molar mass = 106 g] 2 3 B. 248 g of Na O(s) [molar mass = 62 g] 2 C. 240 g of NaOH(s) [molar mass = 40 g] D. 12 g of H (g) [molar mass = 2 g] 2 E. 220 g of CO (g) [molar mass = 44 g] 2 Choose the correct answer from the options given below :

1B, D, and E only

2A, B, and C only

3A, B, and D only

4B, C, and D only

NTA Answer: Option 3(final)

NEET 2025

Sugar ‘X’ A. is found in honey B. is a keto sugar C. exists in  and  – anomeric forms. D. Is laevorotatory. ‘X’ is :

1Sucrose

2D-Glucose

3D-Fructose

4Maltose

NTA Answer: Option 3(final)

NEET 2025

For the reaction A(g)  2B(g), the backward reaction rate constant is higher than the forward reaction rate constant by a factor of 2500, at 1000 K. [Given : R = 0.0831 L atm mol–1 K–1] K for the reaction at 1000 K is P

10.021

283.1

32.077 × 105

40.033

NTA Answer: Option 4(final)

NEET 2024Revised key

C [A] = [B] = [C] = 2 × 10–3 M. Then, which of the following is correct?

1Reaction is at equilibrium.

2Reaction has a tendency to go in forward direction.

3Reaction has a tendency to go in backward direction.

4Reaction has gone to completion in forward direction.

NTA Answer: Option 3(revised_final)

NEET 2023

The relation between n , (n = the number of permissible values of magnetic quantum number (m)) for a m m given value of azimuthal quantum number (l), is

1l = 2n + 1

2n = 2l2 + 1 m m n –1

3n = l + 2

4l  m m 2

NTA Answer: Option 4(final)

NEET 2023

Amongst the given options which of the following molecules/ ion acts as a Lewis acid?

1H O

2BF 2 3

3OH–

4NH 3

NTA Answer: Option 2(final)

NEET 2023

Which complex compound is most stable?

1CoNH  NO    3 3 3 3

2CoCl en NO  2 2 3

3CoNH   SO   3 6 2 4 3

4CoNH  H OBrNO   3 4 2  3 2

NTA Answer: Option 2(final)

NEET 2022

Choose the correct statement:

1Both diamond and graphite are used as dry lubricants.

2Diamond and graphite have two dimensional network.

3Diamond is covalent and graphite is ionic.

4Diamond is sp3 hybridised and graphite is sp2 hybridized.

NTA Answer: Option 4(final)

NEET 2022

Which one is not correct mathematical equation for Dalton’s Law of partial pressure? Here p = total pressure of gaseous mixture

1p =p, where  = mole fraction of ith gas in gaseous mixture i i i i p = pressure of ith gas in pure state i

2p = p + p + p 1 2 3 RT RT RT

3p=n +n +n 1 V 2 V 3 V

4p = p, where p = partial pressure of ith gas i i i  = mole fraction of ith gas in gaseous mixture i

NTA Answer: Option 1(final)

NEET 2022

A 10.0 L flask contains 64 g of oxygen at 27ºC. (Assume O 2 gas is behaving ideally). The pressure inside the flask in bar is (Given R = 0.0831 L bar K–1 mol–1)

14.9

22.5

3498.6

449.8

NTA Answer: Option 1(final)

NEET 2022

- 19 - 3 O 2 ( g ) 2 O 3 ( g ) for the above reaction at 298 K, K is found to be 3.0 × 10–59. If the concentration of O at equilibrium is C 2 0.040 M then concentration of O in M is 3

11.2 × 1021

24.38 × 10–32

31.9 × 10–63

42.4 × 1031

NTA Answer: Option 2(final)

NEET 2022

Find the emf of the cell in which the following reaction takes place at 298 K - 20 - N (i s ) + 2 A g + ( 0 .0 0 1 M ) → N 2i + ( 0 .0 0 1 M ) + 2 A g ( s )  G iv e n th a t E c e ll = 1 0 .5 V , 2 .3 0 3 F R T = 0 .0 5 9 a t 2 9 8 K 

11.05 V

21.0385 V

31.385 V

40.9615 V Answer (NA)

NTA Answer: Option 5(final)

NEET 2021

BF is planar and electron deficient compound. 3 Hybridization and number of electrons around the central atom, respectively are : sp2 sp3

1and 8

2and 4 sp3 sp2

3and 6

4and 6

NTA Answer: Option 4(final)

NEET 2021

Among the following alkaline earth metal halides, one which is covalent and soluble in organic solvents is :

1Beryllium chloride

2Calcium chloride

3Strontium chloride

4Magnesium chloride

NTA Answer: Option 1(final)

NEET 2021

Choose the correct option for the total pressure (in atm.) in a mixture of 4 g O and 2 g H confined in 2 2 a total volume of one litre at 0°C is : mol–1K–1, [Given R = 0.082 L atm T = 273 K]

126.02

22.518

32.602

425.18

NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Convert between K_p and K_c using K_p = K_c (RT)^Δn. Identify Δn carefully.

Multi StepMedium

Common distractors

wrong delta n sign

Counts moles incorrectly

Predict equilibrium shift on changing concentration, P, T, or adding catalyst.

InterpretationMedium

Common distractors

treats catalyst as shifting

Believes catalyst shifts equilibrium

Compute pH of weak acid/base solution using Ka or Kb. Approximation: [H+] ≈ √(Ka·C).

Multi StepMedium

Common distractors

ignores weak vs strong

Treats weak acid as fully dissociated

Compute solubility from K_sp or vice versa. Apply common-ion effect when relevant.

Multi StepMedium

Common distractors

forgets stoichiometric coefficient power

Uses [M+]^1 [X-]^1 for M(X)_2 salt

Sources

NCERT refs: Class 11 Chemistry Chapter 7, p.38

Test yourself on this topic with real past-paper questions:

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Solubility Product

Lesson

Practice MCQs

Quick recall before you leave

Ksp general expression

Q vs Ksp precipitation rule

Worked Example

Before solving, remember these

Formulas

Valid when

Valid when

Valid when

Valid when

Valid when

Exam Traps & Common Mistakes

When it triggers

How to avoid

When it triggers

How to avoid

When it triggers

How to avoid

Correction

Correction

Correction

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Common distractors

Common distractors

Common distractors

Sources

Free NEET study resources