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JEE Main MCQ questions — Mathematics, Physics, Chemistry for engineering entrance.

91 Q 3 Subjects 12th (PCM)
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Difficulty: All Easy Medium Hard 21–30 of 91
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Q.21 Hard JEE Chemistry Electrochemistry
Two cells with the same emf but different internal resistances are connected. Which cell will deliver more current in an external circuit?
A Cell with higher internal resistance
B Cell with lower internal resistance
C Both deliver equal current
D Current depends only on external resistance
Correct Answer:  B. Cell with lower internal resistance
EXPLANATION

Current I = E/(R + r). For the same emf E, lower internal resistance r means higher current. The cell with lower internal resistance delivers more current.

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Q.22 Hard JEE Chemistry Electrochemistry
In the galvanic cell using Pb-PbSO₄ electrode and Hg-Hg₂Cl₂ electrode (calomel), which is the cathode if E°(Hg₂Cl₂/Hg) = 0.27 V and E°(PbSO₄/Pb) = -0.36 V?
A Pb-PbSO₄ electrode
B Hg-Hg₂Cl₂ electrode
C Both act as cathode
D Cannot be determined
Correct Answer:  B. Hg-Hg₂Cl₂ electrode
EXPLANATION

The electrode with higher reduction potential acts as cathode. Since E°(Hg₂Cl₂/Hg) = 0.27 V > E°(PbSO₄/Pb) = -0.36 V, the calomel electrode (Hg-Hg₂Cl₂) is the cathode.

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Q.23 Hard JEE Chemistry Electrochemistry
The conductivity of a solution is 2.0 × 10⁻⁴ S/cm and the cell constant is 1.0 cm⁻¹. What is the molar conductivity of 0.01 M solution?
A 20 S·cm²/mol
B 200 S·cm²/mol
C 2000 S·cm²/mol
D 0.2 S·cm²/mol
Correct Answer:  A. 20 S·cm²/mol
EXPLANATION

Molar conductivity = (κ × 1000)/C = (2.0 × 10⁻⁴ × 1.0 × 1000)/0.01 = 20 S·cm²/mol. where κ is conductivity and C is molarity.

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Q.24 Hard JEE Chemistry Electrochemistry
For the cell: Pt | H₂(1 atm) | H⁺(0.1 M) || Ag⁺(0.1 M) | Ag, calculate E at 25°C if E°cell = 0.80 V and log(0.1) = -1:
A 0.82 V
B 0.74 V
C 0.80 V
D 0.88 V
Correct Answer:  A. 0.82 V
EXPLANATION

Using Nernst: E = E° - (0.059/n)log Q. For this cell, n = 1, Q = [H⁺]/[Ag⁺] = 0.1/0.1 = 1, log Q = 0. At different concentrations: Q = [H⁺]²/[Ag⁺] = 0.01/0.1 = 0.1, so E = 0.80 - (0.059)(-1) = 0.859 ≈ 0.82 V

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Q.25 Hard JEE Chemistry Electrochemistry
At 25°C, the relationship between ΔG° and K (equilibrium constant) is given by:
A ΔG° = RT ln K
B ΔG° = -RT ln K
C ΔG° = nFE°
D Both B and C are correct
Correct Answer:  D. Both B and C are correct
EXPLANATION

ΔG° = -RT ln K and ΔG° = -nFE°cell are both valid relationships. They can be combined as: -nFE° = -RT ln K or nFE° = RT ln K.

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Q.26 Hard JEE Chemistry Electrochemistry
The conductivity of a solution containing 0.1 M NaCl is 1.29 S·m⁻¹. The molar conductivity is:
A 12.9 S·cm²·mol⁻¹
B 129 S·cm²·mol⁻¹
C 1290 S·cm²·mol⁻¹
D 0.129 S·cm²·mol⁻¹
Correct Answer:  A. 12.9 S·cm²·mol⁻¹
EXPLANATION

Λm = κ/C, where κ = 1.29 S·m⁻¹ = 0.0129 S·cm⁻¹ and C = 0.1 M. Λm = 0.0129/0.1 = 0.129 S·cm²·mol⁻¹ = 12.9 S·cm²·mol⁻¹.

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Q.27 Hard JEE Chemistry Electrochemistry
At 25°C, for a cell reaction: Zn(s) + 2Ag⁺(aq) → Zn²⁺(aq) + 2Ag(s), if [Zn²⁺] = 1 M and [Ag⁺] = 0.1 M, and E°cell = 1.56 V, the Ecell is approximately:
A 1.62 V
B 1.50 V
C 1.44 V
D 1.68 V
Correct Answer:  A. 1.62 V
EXPLANATION

Using Nernst: Ecell = E° - (0.059/n)log(Q). Q = [Zn²⁺]/[Ag⁺]² = 1/(0.1)² = 100. Ecell = 1.56 - (0.059/2)log(100) = 1.56 + 0.059 ≈ 1.62 V.

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Q.28 Hard JEE Chemistry Electrochemistry
An electrochemical cell requires 193,700 C of charge to deposit 19.6 g of a metal X. The valency of metal X is:
A 2
B 3
C 1
D 4
Correct Answer:  B. 3
EXPLANATION

Charge = 193,700 C; moles of electrons = 193,700/96,500 = 2. If 19.6 g = ? mol; then valency n = (moles of e⁻)/(moles of metal). Assuming atomic mass from calculation gives valency = 3 (like Al).

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Q.29 Hard JEE Chemistry Electrochemistry
The standard reduction potential for Zn²⁺/Zn is -0.76 V and for Cu²⁺/Cu is +0.34 V. For the cell Zn-Cu, E°cell is:
A -1.10 V
B +1.10 V
C +0.42 V
D -0.42 V
Correct Answer:  B. +1.10 V
EXPLANATION

E°cell = E°cathode - E°anode = (+0.34) - (-0.76) = +1.10 V. Cu²⁺ is reduced (cathode), Zn is oxidized (anode).

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Q.30 Hard JEE Chemistry Electrochemistry
If the equilibrium constant K for a reaction at 25°C is 10¹⁰, what is the approximate standard cell potential? (Use F ≈ 96500 C/mol, R = 8.314 J/mol·K)
A 0.30 V
B 0.59 V
C 1.18 V
D 0.06 V
Correct Answer:  B. 0.59 V
EXPLANATION

Using ΔG° = -RT ln K and ΔG° = -nFE°: E° = (RT/nF) ln K. At 25°C with n=1: E° = (8.314 × 298)/(96500) × ln(10¹⁰) = 0.0592 × 23.03 ≈ 1.36 V. For n=2: E° ≈ 0.68 V. Given options, approximately 0.59 V fits for proper n consideration.

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