Nernst Equation

8 MCQs2 revision cards9-step worked example

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

Lesson

The Nernst equation tells you what happens to a cell's EMF when concentrations deviate from the standard 1 M. It is also where a large fraction of NEET negative marks originate — not from the equation itself, but from plugging in the wrong value of n.

The equation (at 298 K):

E = E° − (0.0591/n) × log₁₀ Q

where E° is the standard cell EMF, n is the number of electrons transferred in the balanced redox equation, and Q is the reaction quotient ([products]/[reactants], each raised to stoichiometric powers; pure solids and liquids excluded).

Standard cell EMF recap: E°_cell = E°_cathode − E°_anode, with both values taken as reduction potentials (NCERT Class 12 Chemistry Chapter 2, page 12).

The high-frequency trap: wrong n. The electron count must come from the balanced overall equation, not from a single half-reaction in isolation. For the Daniel cell (Zn | Zn²⁺ || Cu²⁺ | Cu), each half-reaction involves 2 electrons, so n = 2. But if KMnO₄ acts as oxidising agent in acidic medium, Mn goes from +7 to +2, transferring 5 electrons — and that number changes to 3 (→ MnO₂) in neutral medium or 1 (→ MnO₄²⁻) in strongly basic medium. The medium dictates n, not just the reagent.

Key boundary conditions to memorise:

At standard conditions: Q = 1, log Q = 0, so E = E°.
At equilibrium: E = 0, so E° = (0.0591/n) × log₁₀ K. This links electrochemistry to the equilibrium constant.

Watch-out: NEET distractors routinely offer an answer that uses n = 1 where the balanced equation demands n = 2 (or vice versa). Before substituting into the Nernst equation, write out the balanced redox equation and explicitly count electrons. Every time.

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

In the Nernst equation E = E° − (0.0591/n) log Q at 298 K, what does 'n' represent?

MCQ 2Easy RecallPractice

At equilibrium, the cell potential E in the Nernst equation equals:

MCQ 3Easy RecallPractice

When all species in a galvanic cell are at their standard states (1 M concentration, 298 K), the value of the reaction quotient Q is:

MCQ 4Direct ApplicationPractice

For the cell reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), with E° = +1.10 V, what is the cell EMF when [Cu²⁺] = 0.01 M and [Zn²⁺] = 1.0 M at 298 K?

MCQ 5Direct ApplicationPractice

Given E°(Ag⁺/Ag) = +0.80 V and E°(Cu²⁺/Cu) = +0.34 V, the standard EMF of the cell Cu(s) | Cu²⁺(aq) || Ag⁺(aq) | Ag(s) is:

MCQ 6Direct ApplicationPractice

For the half-reaction MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O in acidic medium, what value of n should be used in the Nernst equation for a cell based on this half-reaction?

MCQ 7CalculationPractice

For the cell Zn | Zn²⁺(0.1 M) || Ag⁺(0.1 M) | Ag, given E°(Zn²⁺/Zn) = −0.76 V and E°(Ag⁺/Ag) = +0.80 V, the cell EMF at 298 K is:

MCQ 8Concept TrapPractice

A student calculates the EMF of a cell involving KMnO₄ as oxidising agent in neutral medium but uses n = 5. The actual product in neutral medium is MnO₂ (not Mn²⁺). The student's calculated EMF will be:

Quick recall before you leave

Worked Example

Pattern: Nernst equation — compute non-standard cell EMF (NEET pattern: nernst equation problem).

1
Given
Cell: Zn(s) | Zn²⁺(aq, 0.001 M) || Cu²⁺(aq, 0.1 M) | Cu(s) E°(Zn²⁺/Zn) = −0.76 V E°(Cu²⁺/Cu) = +0.34 V Temperature = 298 K
2
Required
Calculate the cell EMF (E) at the given non-standard concentrations.
3
Concept
The Nernst equation adjusts the standard EMF for the effect of concentration. The reaction quotient Q depends on the balanced overall equation.
4
Formula
E°_cell = E°_cathode − E°_anode E = E° − (0.0591/n) × log₁₀ Q
5
Substitution setup
Balanced reaction: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) Electrons transferred: n = 2 (Zn → Zn²⁺ + 2e⁻) E° = 0.34 − (−0.76) = 1.10 V Q = [Zn²⁺]/[Cu²⁺] = 0.001/0.1 = 0.01
6
Calculation
log₁₀(0.01) = −2 E = 1.10 − (0.0591/2) × (−2) E = 1.10 − (0.02955) × (−2) E = 1.10 + 0.0591 E = 1.1591 V Note on exact values: The coefficient 2 in n = 2 is a counting integer (electrons per balanced equation) and does not limit significant figures.
7
Final answer
E = 1.16 V (rounded to 3 significant figures, matching the precision of the given E° values). The factor 0.0591 is derived from (RT ln 10)/F at 298 K and is conventionally treated as exact for NEET calculations.
8
Common trap
Using n = 1 instead of n = 2 would double the correction term: (0.0591/1) × (−2) = −0.1182, giving E = 1.10 + 0.1182 = 1.22 V — a distractor commonly seen in NEET options (trap: trap: nernst n electrons negmark; mistake: mistake: nernst wrong n).
9
Similar NEET-style question
For the cell Fe(s) | Fe²⁺(0.01 M) || Ag⁺(1.0 M) | Ag(s), with E°(Fe²⁺/Fe) = −0.44 V and E°(Ag⁺/Ag) = +0.80 V, calculate E at 298 K. (Answer: n = 2, Q = 0.01/1² = 0.01, E = 1.24 + 0.0591 = 1.30 V.) ---

Before solving, remember these

Formula

Nernst equation

E = E° - (RT/nF) ln Q = E° - (0.0591/n) log Q at 298 K. Q = reaction quotient. At equilibrium E=0 and Q=K, giving E° = (0.0591/n) log K.

-- NCERT Class 12 Chemistry, Ch. 2, p. 12

Formulas

Cell EMF

E_{cell}^{\circ} = E_{cath}^{\circ} - E_{anode}^{\circ}

Both as reduction potentials. E°_cell > 0 → spontaneous.

Symbol	Quantity	SI Unit
E°_cell	standard cell EMF	V
E°_red	reduction potential	V

Valid when

Standard conditions (1 M, 1 bar, 298 K)
Both half-reactions as reductions

Faraday's law of electrolysis

m = \frac{M I t}{n F}

Mass deposited at electrode. M = molar mass; I = current; t = time; n = electrons per ion.

Symbol	Quantity	SI Unit
m	mass deposited	g
M	molar mass	g/mol
I	current	A
t	time	s
n	electrons per ion	-
F	Faraday	C/mol

Valid when

Steady current
Single product

ΔG from EMF

Δ G = - n F E

Connection between thermodynamics and electrochemistry. F = 96485 C/mol.

Symbol	Quantity	SI Unit
ΔG	Gibbs energy change	J
n	electrons transferred	-
F	Faraday 96485	C/mol
E	cell EMF	V

Valid when

Single redox process

Molar conductivity

Λ_{m} = \frac{1000 κ}{C}

Molar conductivity from specific conductance. Increases with dilution as more ions are free.

Symbol	Quantity	SI Unit
Λ_m	molar conductivity	S cm^2/mol
κ	specific conductance	S/cm
C	molarity	mol/L

Valid when

Aqueous solution
Single electrolyte

Nernst equation

E = E^{\circ} - \frac{0.0591}{n} lo g Q

Cell potential at non-standard conditions. n = electrons transferred. At equilibrium E=0, Q=K.

Symbol	Quantity	SI Unit
E	cell potential	V
E°	standard	V
n	electrons	-
Q	reaction quotient	-

Valid when

298 K (else use RT/F)
Single redox process

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

Nernst n electrons cumulative error

Category: Negative Marking

Multi-step Nernst problem: identify electrons, write Q correctly, plug into 0.0591/n. Each sub-step has factor errors.

When it triggers

Cell EMF problem at non-standard conditions.

How to avoid

Step-by-step: (1) write balanced redox; (2) count n electrons; (3) compute Q from concentrations; (4) plug into Nernst. Verify by checking limits: at standard conditions Q=1, log Q=0, E=E°.

Trap

KMnO4 oxidation product depends on medium

Category: Inorganic Exception

Student assumes Mn²⁺ is the product regardless of medium. Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).

When it triggers

Question gives KMnO4 oxidation in unspecified or specific medium.

How to avoid

Always check medium. In acidic: Mn(+7) → Mn(+2). In neutral: → Mn(+4) (MnO₂). In basic: → Mn(+6) (manganate). The number of electrons (n) in Nernst calculations depends accordingly.

Mistake

Uses n=1 in Faraday's law for ions like Cu²⁺ (actual n=2) or Al³⁺ (n=3).

Root cause: concept gap

Correction

n = number of electrons per ion to deposit. Cu²⁺ + 2e⁻ → Cu: n=2. Al³⁺ + 3e⁻: n=3. m = MIt/(nF).

Mistake

Uses incorrect electron count n in Nernst equation.

Root cause: formula misuse

Correction

n = electrons transferred per balanced redox equation. For Cu²⁺ + 2e⁻ → Cu: n=2. For Mn(VII) → Mn(II): n=5.

Past Year Questions

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

NEET 2025

If the rate constant of a reaction is 0.03 s–1, how much time does it take for 7.2 mol L–1 concentration of the reactant to get reduced to 0.9 mol L–1? (Given: log 2 = 0.301)

121.0 s

269.3 s

323.1 s

4210 s

NTA Answer: Option 2(final)

NEET 2025

Consider the following compounds : KO , H O and H SO 2 2 2 2 4 The oxidation state of the underlined elements in them are, respectively,

1+4, –4, and +6

2+1, –1, and +6

3+2, –2, and +6

4+1, –2, and +4

NTA Answer: Option 2(final)

NEET 2025

Match List-I with List-II List-I List-II (Example) (Type of Solution) A. Humidity I. Solid in solid B. Alloys II. Liquid in gas C. Amalgams III. Solid in gas D. Smoke IV. Liquid in solid Choose the correct answer from the options given below:

1A-III, B-II, C-I, D-IV

2A-II, B-IV, C-I,D-III

3A-II, B-I, C-IV, D-III

4A-III, B-I, C-IV, D-II

NTA Answer: Option 3(final)

NEET 2024Revised key

Given below are two statements: Statement I : The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane Statement II : When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point. In the light of the above statements, choose the most appropriate answer from the options given below:

1Both Statement I and Statement II are correct.

2Both Statement I and Statement II are incorrect.

3Statement I is correct but Statement II is incorrect.

4Statement I is incorrect but Statement II is correct.

NTA Answer: Option 1(revised_final)

NEET 2024Revised key

Match List I with List II. List I List II (Conversion) (Number of Faraday required) A. 1 mol of H O to O I. 3F 2 2 B. 1 mol of MnO− 4 to Mn2+ II. 2F C. 1.5 mol of Ca from molten CaCl III. 1F 2 D. 1 mol of FeO to Fe O IV. 5F 2 3 Choose the correct answer from the options given below:

1A-II, B-IV, C-I, D-III

2A-III, B-IV, C-I, D-II

3A-II, B-III, C-I, D-IV

4A-III, B-IV, C-II, D-I

NTA Answer: Option 1(revised_final)

NEET 2024Revised key

Mass in grams of copper deposited by passing 9.6487 A current through a voltmeter containing copper sulphate solution for 100 seconds is (Given : Molar mass of Cu : 63 g mol–1, 1 F = 96487 C)

13.15 g

20.315 g

331.5 g

40.0315 g

NTA Answer: Option 2(revised_final)

NEET 2024Revised key

The products A and B obtained in the following reactions, respectively, are 3ROH + PCl → RCl + A 3 ROH + PCl → RCl + HCl + B 5

1POCl and H PO 3 3 3

2POCl and H PO 3 3 4

3H PO and POCl 3 4 3

4H PO and POCl 3 3 3

NTA Answer: Option 4(revised_final)

NEET 2023

The conductivity of centimolar solution of KCl at 25°C is 0.0210 ohm–1 cm–1 and the resistance of the cell containing the solution at 25°C is 60 ohm. The value of cell constant is

13.28 cm–1

21.26 cm–1

33.34 cm–1

41.34 cm–1

NTA Answer: Option 2(final)

NEET 2023

The number of  bonds,  bonds and lone pair of electrons in pyridine, respectively are:

112, 3, 0

211, 3, 1

312, 2, 1

411, 2, 0

NTA Answer: Option 2(final)

NEET 2022

At 298 K, the standard electrode potentials of Cu2+ / Cu, Zn2+ / Zn, Fe2+ / Fe and Ag+ / Ag are 0.34 V, –0.76 V, –0.44 V and 0.80 V, respectively. On the basis of standard electrode potential, predict which of the following reaction cannot occur?

12CuSO (aq) + 2Ag(s) → 2Cu(s) + Ag SO (aq) 4 2 4

2CuSO (aq) + Zn(s) → ZnSO (aq) + Cu(s) 4 4

3CuSO (aq) + Fe(s) → FeSO (aq) + Cu(s) 4 4

4FeSO (aq) + Zn(s) → ZnSO (aq) + Fe(s) 4 4

NTA Answer: Option 1(final)

NEET 2022

Which of the following statement is not correct about diborane?

1Both the Boron atoms are sp2 hybridised.

2There are two 3-centre-2-electron bonds.

3The four terminal B-H bonds are two centre two electron bonds.

4The four terminal Hydrogen atoms and the two Boron atoms lie in one plane.

NTA Answer: Option 1(final)

NEET 2022

Given below are half cell reactions: - 13 - M n O −4 + 8 H + + 5 e − → M n 2 + + 4 H 2 O E ºM n 2 + /M n O −4 = − 1 .5 1 0 V 1 2 O 2 + 2 H + + 2 e − → H 2 O Eº =+1.223V O2/H2O Will the permanganate ion, M n O –4 liberate O from water in the presence of an acid? 2

1No, because E cell =−2.733V

2Yes, because E c e ll = + 0 .2 8 7 V

3No, because E c e ll = – 0 .2 8 7 V

4Yes, because E c e ll = + 2 .7 3 3 V

NTA Answer: Option 2(final)

NEET 2022

If radius of second Bohr orbit of the He+ ion is 105.8 pm, what is the radius of third Bohr orbit of Li2+ ion?

1158.7 Å

2158.7 pm

315.87 pm

41.587 pm

NTA Answer: Option 2(final)

NEET 2021

Which one among the following is the correct option for right relationship between C and C for one mole P V of ideal gas?

1C = RC V P

2C + C = R P V

3C – C = R P V

4C = RC P V

NTA Answer: Option 3(final)

NEET 2021

The molar conductance of NaCl, HCl and CH COONa 3 at infinite dilution are 126.45, 426.16 and cm2 mol–1 91.0 S respectively. The molar conductance of CH COOH at infinite dilution is. 3 Choose the right option for your answer. cm2 mol–1

1540.48 S cm2 mol–1

2201.28 S cm2 mol–1

3390.71 S cm2 mol–1

4698.28 S

NTA Answer: Option 3(final)

NEET 2021

Which one of the following polymers is prepared by addition polymerisation?

1Dacron

2Teflon

3Nylon-66

4Novolac

NTA Answer: Option 2(final)

NEET 2021

The molar conductivity of 0.007 M acetic acid is 20 cm2 mol–1. S What is the dissociation constant of acetic acid? Choose the correct option. Λ° =350Scm2 mol−1  H+   Λ° mol−1 =50Scm2  CHCOO−  3 2.50×10−5 molL−1

11.75×10−4 molL−1

22.50×10−4 molL−1

31.75×10−5 molL−1

4Answer

NTA Answer: Option 4(final)

How NEET usually asks this

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

Compute Λ_m from κ and C. Or compute Λ°_m of weak electrolyte from strong electrolyte values via Kohlrausch.

Direct ApplicationMedium

Common distractors

inverts c multiplier

Uses Λ = κC instead of κ/C

forgets 1000 factor

Drops 1000 in unit conversion

Apply Nernst equation to compute non-standard cell EMF given Q. n = electrons transferred.

Multi StepMedium

Common distractors

wrong n electrons

Uses incorrect electron count from half-reactions

Sources

NCERT refs: Class 12 Chemistry Chapter 2, p.12

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

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Nernst Equation

Lesson

Practice MCQs

Quick recall before you leave

Nernst equation (298 K)

Standard cell EMF

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

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Common distractors

Sources

Free NEET study resources