Subtopics - Ionic Equilibrium (NEET)
Seven topic blocks: acid-base theories (Bronsted-Lowry and Lewis with conjugate pairs and acid/base types), pH scale and ionic product of water, Ostwald's dilution law for weak electrolytes, common ion effect, salt hydrolysis with pH formulas for four salt categories, buffer solutions with Henderson-Hasselbalch equation and buffer capacity, and solubility product with selective precipitation rules.
1) Acid and Bases
Covers three acid-base theories in order of generality: Arrhenius (limited to aqueous solutions), Bronsted-Lowry (proton transfer, applicable to non-aqueous media), and Lewis (electron pair donation/acceptance, the most general). Includes conjugate acid-base pairs, types of Lewis acids (incomplete octet, cations, empty d-orbitals, multiple bonds), and types of Lewis bases (lone pair donors, anions).
2) Strength of Acids and Bases and pH scales
Defines acidic and basic strength in terms of H+ and OH- ion donation. Introduces the pH scale (Sorenson), ionic product of water Kw = [H+][OH-] = 10^-14 at 298 K, and the relation pH + pOH = pKw = 14 at 298 K. Covers self-ionisation of water and the temperature dependence of Kw.
3) Hydrolysis of salts and their pH
When a salt dissolves in water and its constituent ions react with H+ or OH- from water to regenerate the parent weak acid or base, the process is called hydrolysis. Three salt types hydrolyse: weak acid + strong base (basic solution), strong acid + weak base (acidic solution), and weak acid + weak base (pH depends on Ka vs Kb). A fourth type, strong acid + strong base, does not hydrolyse.
4) Buffer Solutions, Mechanism of Action and their pH
A buffer solution resists pH changes upon addition of small amounts of acid or base and upon dilution. Contains a weak acid/base and its conjugate. Acidic buffer: weak acid + salt of that acid. Basic buffer: weak base + salt of that base. Salt buffer: salt of weak acid with weak base. The Henderson-Hasselbalch equation gives pH directly.
5) Solubility and Solubility product
Defines solubility as the maximum amount of salt that dissolves at a given temperature. For a sparingly soluble salt AxBy: Ksp = x^x times y^y times s^(x+y), where s is molar solubility. Covers five cases: solubility in pure water, with common ion, with complex formation, for salts of weak acid (hydrolysis correction), and simultaneous solubility of two salts.
6) Selective precipitation
Compares the ionic product (IP) with Ksp to predict whether precipitation occurs. If IP > Ksp, precipitation continues until IP = Ksp. If IP < Ksp, no precipitation. Used in qualitative analysis to selectively precipitate one ion while keeping another in solution.
Ionic Equilibrium Download Notes & Weightage Plan
For each topic in the Ionic Equilibrium chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Three acid-base theories in order of generality: Arrhenius, Bronsted-Lowry, and Lewis. Conjugate acid-base pairs, types of Lewis acids and bases.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Lewis acid identification: BF3, AlCl3, SiF4 are Lewis acids because of electron deficiency or empty d-orbitals. NH3 is a Lewis base (lone pair donor). BF3 is the most tested Lewis acid in NEET.
- High-risk Area: Students confuse Lewis acids with Bronsted acids. CO2 is a Lewis acid (accepts electron pair via pi bond shift) but is NOT a Bronsted acid (it does not donate H+). NEET uses CO2 as a Lewis acid option to test this distinction.
- Best Practice Style: For Lewis acid questions: check if the species can accept an electron pair. Look for incomplete octets, positive charges, or empty orbitals. If yes, it is a Lewis acid.
Strength of Acids and Bases and pH scales
Defines pH, pOH, Kw, and the pH scale. Covers self-ionisation of water, the temperature dependence of Kw, and the common ion effect on weak electrolyte dissociation.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: For strong acids with concentration greater than 10^-6 M: pH = -log C. For very dilute acids (10^-7 to 10^-8 M): add water contribution. Common ion effect: know that adding NaCl to AgCl equilibrium or CH3COONa to CH3COOH suppresses dissociation.
- High-risk Area: Very dilute strong acid (e.g., 10^-8 M HCl) gives pH = 6.98, not 8. Students who blindly apply pH = -log(10^-8) = 8 conclude the acid is basic, which is absurd. The water autoionisation contribution (10^-7 M) must be included.
- Best Practice Style: If [H+] from the acid is less than 10^-6 M, add the water contribution: total [H+] = [H+]acid + [H+]water. Solve the quadratic if needed.
Hydrolysis of salts and their pH
Derives pH formulas for three types of hydrolysable salts: weak acid + strong base (basic), strong acid + weak base (acidic), and weak acid + weak base (depends on Ka vs Kb). Strong acid + strong base salts do not hydrolyse.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Quick classification: see Na2CO3 or CH3COONa? Weak acid + strong base, pH > 7. See NH4Cl? Strong acid + weak base, pH < 7. See NaCl? No hydrolysis, pH = 7. Then apply the corresponding formula.
- High-risk Area: Swapping the WA+SB formula with the SA+WB formula. Remember: WA+SB has PLUS signs (pH = 7 + ... + ...) because the solution is basic (pH > 7). SA+WB has MINUS signs (pH = 7 - ... - ...) because the solution is acidic (pH < 7). The signs match the expected side of 7.
- Best Practice Style: First determine the salt type. Then recall: basic solution has + signs, acidic solution has - signs. Substitute pKa or pKb and log C. Verify: is the final pH on the correct side of 7?
Buffer Solutions, Mechanism of Action and their pH
Defines buffer solutions and explains their mechanism of resisting pH change. Covers acidic buffers (weak acid + conjugate base salt), basic buffers (weak base + conjugate acid salt), and salt buffers. Henderson-Hasselbalch equation and buffer capacity.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Henderson-Hasselbalch equation: pH = pKa + log([salt]/[acid]). When equal concentrations of acid and salt are mixed, pH = pKa. This is the single most important result for NEET buffer questions.
- High-risk Area: Confusing [salt] with [acid] in the Henderson-Hasselbalch ratio. The ratio is always conjugate base (salt) over acid. Inverting it gives pH = pKa - log([salt]/[acid]) = pKa + log([acid]/[salt]), which is the wrong formula.
- Best Practice Style: Write the Henderson-Hasselbalch equation with labels: pH = pKa + log(BASE/ACID). The base (conjugate base or salt) is always in the numerator.
Solubility and Solubility product
Relates molar solubility to the solubility product Ksp. Covers the general formula Ksp = x^x y^y s^(x+y) for AxBy salts. Five cases: pure water, common ion, complex formation, hydrolysis correction, and simultaneous solubility.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Two high-yield moves: (1) For AB2 type (e.g., CaF2): Ksp = 4s^3, so s = (Ksp/4)^(1/3). (2) IP vs Ksp comparison: if calculated IP exceeds Ksp, precipitation occurs. These two skills answer most NEET Ksp questions.
- High-risk Area: Forgetting the coefficient multipliers in Ksp expressions. For PbCl2 (PbCl2 to Pb2+ + 2Cl-): Ksp = s times (2s)^2 = 4s^3, not s^3. Missing the factor of 4 gives s that is too large by a factor of (4)^(1/3) = 1.587. NEET places the wrong value as a distractor.
- Best Practice Style: Always write the dissociation equation with stoichiometric coefficients. Each ion concentration = coefficient times s. Then substitute into Ksp expression and simplify.
Uses the IP vs Ksp comparison to predict and control precipitation of specific ions from a mixture. Foundational to qualitative inorganic analysis.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: When two solutions are mixed, first calculate the new concentrations (account for dilution), then calculate IP, then compare with Ksp. If IP > Ksp, precipitation occurs.
- High-risk Area: Forgetting to account for dilution when solutions are mixed. If 500 mL of 0.005 M AgNO3 is mixed with 500 mL of 0.001 M KCl, the total volume is 1000 mL, so [Ag+] = 0.005 times 500/1000 = 0.0025 M, not 0.005 M. Using undiluted concentrations gives an IP that is 4 times too large.
- Best Practice Style: Step 1: calculate diluted concentrations. Step 2: calculate IP. Step 3: compare with Ksp. Three steps, no shortcuts.
Ionic Equilibrium Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Ionic Equilibrium chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Using the wrong pH formula for the salt type: The pH formula for weak acid + strong base salt has PLUS signs (pH = 7 + ...) while the formula for strong acid + weak base salt has MINUS signs (pH = 7 - ...). Swapping them gives pH on the wrong side of 7: a basic salt appears acidic and vice versa.
- Forgetting that WA+WB salt pH is independent of concentration: For salts of weak acid with weak base, pH = (1/2)pKw + (1/2)pKa - (1/2)pKb with no log C term. Students who add a concentration term get a wrong answer.
0.1 M CH3COONa (weak acid + strong base). pH = 7 + (1/2)(4.75) + (1/2)(log 0.1) = 7 + 2.375 + (-0.5) = 8.875. If student uses the SA+WB formula: pH = 7 - 2.375 - (-0.5) = 5.125, which incorrectly shows acidic solution for a basic salt.
How NEET Frames The Trap
NEET asks for pH of a salt solution. The distractors are calculated using the wrong formula for the salt type.
Q. The pH of 0.01 M NH4Cl solution is (Kb of NH4OH = 1.8 x 10^-5)
A. 6.13 B. 7.87 C. 7.00 D. 4.75
Trick: NH4Cl = strong acid + weak base, so pH < 7. pKb = -log(1.8 x 10^-5) = 4.74. pH = 7 - (1/2)(4.74) - (1/2)log(0.01) = 7 - 2.37 - (-1) = 7 - 2.37 + 1 = 5.63. Wait, the correct calculation: pH = 7 - (1/2)(4.74) - (1/2)(log 0.01) = 7 - 2.37 - (-1) = 5.63. Option A (6.13) is closest. Option B uses the WA+SB formula giving pH > 7 for an acidic salt.
Mistake Snapshot (What Students Do Wrong)
- Inverting the [salt]/[acid] ratio in Henderson-Hasselbalch: pH = pKa + log([salt]/[acid]). Inverting to [acid]/[salt] changes the sign of the log term. If [salt] > [acid], the correct formula gives pH > pKa, but the inverted formula gives pH < pKa.
- Using molarity instead of moles when volumes differ: When acid and salt solutions of different volumes are mixed, the concentrations change. Using initial molarities without adjusting for final volume gives the wrong ratio. Since both acid and salt are in the same final volume, the volume cancels and only moles matter.
500 mL of 0.1 M CH3COOH mixed with 500 mL of 0.2 M CH3COONa. pKa = 4.75. Moles acid = 0.05, moles salt = 0.10. Ratio = 0.10/0.05 = 2. pH = 4.75 + log 2 = 4.75 + 0.30 = 5.05. If student uses molarities directly without volume: pH = 4.75 + log(0.2/0.1) = 5.05. Same answer here because equal volumes, but if volumes differ (say 500 mL acid + 250 mL salt): moles acid = 0.05, moles salt = 0.05, ratio = 1, pH = 4.75. Using molarities: log(0.2/0.1) = log 2, giving wrong pH = 5.05.
How NEET Frames The Trap
NEET gives different volumes of acid and salt solutions. Students who use molarities instead of moles after mixing get the wrong ratio.
Q. What is the pH of a buffer made by mixing 200 mL of 0.1 M CH3COOH with 300 mL of 0.1 M CH3COONa? (pKa = 4.75)
A. 4.93 B. 4.75 C. 5.05 D. 4.57
Trick: Moles acid = 0.02, moles salt = 0.03. Ratio = 0.03/0.02 = 1.5. pH = 4.75 + log(1.5) = 4.75 + 0.176 = 4.93 (Option A). Option B ignores the ratio (assumes equal). Option C uses molarities (both 0.1) giving log 1 = 0. Option D inverts the ratio.
Mistake Snapshot (What Students Do Wrong)
- Forgetting stoichiometric coefficient multipliers in Ksp: For CaF2 to Ca2+ + 2F-: [F-] = 2s, so Ksp = s(2s)^2 = 4s^3. Students who write Ksp = s^3 miss the factor of 4 and get solubility that is 4^(1/3) = 1.587 times too large.
- Not accounting for dilution in precipitation problems: When two solutions are mixed, the total volume increases and all concentrations decrease proportionally. Using original concentrations instead of diluted ones gives an ionic product that is too large.
Ksp of PbCl2 = 1.6 x 10^-5. PbCl2 to Pb2+ + 2Cl-. Ksp = s times (2s)^2 = 4s^3. s = (Ksp/4)^(1/3) = (4 x 10^-6)^(1/3) = 1.587 x 10^-2 M. If student writes Ksp = s^3: s = (1.6 x 10^-5)^(1/3) = 2.52 x 10^-2 M, which is 59% too large. NEET places this as a distractor.
How NEET Frames The Trap
NEET gives Ksp and asks for solubility. The incorrect answer calculated without the coefficient multiplier is always one of the four options.
Q. The solubility product of Ag2CrO4 is 1.1 x 10^-12. What is the solubility in mol/L?
A. 6.5 x 10^-5 B. 1.03 x 10^-4 C. 1.1 x 10^-4 D. 5.5 x 10^-7
Trick: Ag2CrO4 to 2Ag+ + CrO4^2-. [Ag+] = 2s, [CrO4^2-] = s. Ksp = (2s)^2 times s = 4s^3. s = (Ksp/4)^(1/3) = (2.75 x 10^-13)^(1/3) = 6.5 x 10^-5 (Option A). Option B uses Ksp = s^3 (no factor 4). Option D uses Ksp = 2s^2.
Mistake Snapshot (What Students Do Wrong)
- Getting pH > 7 for a strong acid solution: For 10^-8 M HCl, blindly applying pH = -log(10^-8) = 8 gives a basic pH for an acid, which is physically impossible. The water autoionisation contribution (10^-7 M H+) must be added.
- Ignoring that very dilute acids still produce acidic solutions: Any acid in water makes [H+] > 10^-7, giving pH < 7. The correct calculation for 10^-8 M HCl: total [H+] approximately 1.05 x 10^-7, so pH approximately 6.98.
10^-8 M HCl. Incorrect: pH = -log(10^-8) = 8 (basic? impossible for an acid!). Correct: total [H+] = 10^-8 + [H+]water. From Kw = [H+][OH-] and charge balance: [H+]^2 - 10^-8[H+] - 10^-14 = 0. [H+] = 1.05 x 10^-7. pH = 6.98. The acid gives pH just below 7, as expected.
How NEET Frames The Trap
NEET asks for pH of a very dilute strong acid (10^-7 to 10^-9 M). The distractor pH = 8 catches students who do not account for water autoionisation.
Q. What is the pH of 10^-8 M HCl solution?
A. 6.98 B. 8.00 C. 7.00 D. 6.00
Trick: Total [H+] = 10^-8 (from HCl) + approximately 10^-7 (from water) = approximately 1.05 x 10^-7. pH = -log(1.05 x 10^-7) = 6.98 (Option A). Option B directly uses -log(10^-8). Option C is pure water pH. Option D is -log(10^-6).
Mistake Snapshot (What Students Do Wrong)
- Confusing Lewis acids with Bronsted acids: CO2 and SO2 are Lewis acids (accept electron pairs via pi bond shifting) but are NOT Bronsted acids (they do not donate H+). Students who think acid = H+ donor miss CO2 as a Lewis acid.
- Thinking BF3 is not an acid because it has no H+: BF3 has an incomplete octet on boron (6 electrons) and readily accepts an electron pair. It is the most classic Lewis acid. Students conditioned by Bronsted theory may not recognise it.
Which of the following is a Lewis acid? (a) NH3 (b) BF3 (c) H2O (d) C2H5OH. Answer: BF3 (Option B). BF3 has only 6 electrons around B and can accept an electron pair. NH3, H2O, and C2H5OH all have lone pairs and are Lewis bases.
How NEET Frames The Trap
NEET gives four molecules and asks which is a Lewis acid. Three options are Lewis bases (lone pair donors) and one is the Lewis acid (electron deficient or has empty orbitals).
Q. Which of the following acts as a Lewis acid?
A. BF3 B. NH3 C. H2O D. (CH3)3N
Trick: BF3 (Option A) has an incomplete octet on boron (6 valence electrons). It accepts an electron pair to complete its octet. NH3 (lone pair on N), H2O (two lone pairs on O), and (CH3)3N (lone pair on N) are all Lewis bases.