Calculate emf of the following cell at 298 K : Cr(s) Cr³+(aq)(0.1\ M) Fe²+(aq)(0.01\ M) Fe(s) (Given : E^oCr³+/Cr = – 0·74 V, E^oFe²+/Fe = – 0·44 V, [log 10 = 1])

ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2026: question 22

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerElectrochemistry

Calculate emf of the following cell at 298 K :

$Cr (s) ∣C r^{3 +} (aq) (0.1 M) ∣∣F e^{2 +} (aq) (0.01 M) ∣Fe (s)$

(Given : $E_{C r^{3 +} / C r}^{o}$ = – 0·74 V, $E_{F e^{2 +} / F e}^{o}$ = – 0·44 V, [log 10 = 1])

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Balance the electrons: Cr loses 3 and Fe²⁺ gains 2, so find the LCM for $n$ .

Hint 2

In the Nernst equation, raise each ion's concentration to the power of its stoichiometric coefficient.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

Cell reaction: $2Cr (s) + 3F e^{2 +} (aq) \to 2C r^{3 +} (aq) + 3Fe (s)$ , so $n = 6$ .
½ mark
$E_{ce l l}^{\circ} = E_{c a t h o d e}^{\circ} - E_{an o d e}^{\circ} = (- 0.44) - (- 0.74)$
½ mark
$E_{ce l l}^{\circ} = 0.30 V$
1 mark
Nernst equation: $E_{ce l l} = E_{ce l l}^{\circ} - \frac{0.059}{6} lo g \frac{[ C r ^{3 +} ] ^{2}}{[ F e ^{2 +} ] ^{3}} = 0.30 - \frac{0.059}{6} lo g \frac{( 0.1 ) ^{2}}{( 0.01 ) ^{3}}$
$= 0.30 - \frac{0.059}{6} lo g 1 0^{4} = 0.30 - \frac{0.236}{6} = 0.30 - 0.0393$
$E_{ce l l} = 0.2607 V \approx 0.26 V$
1 mark

Answer: $E_{ce l l} \approx 0.26$ V

Where the marks are

Balanced cell reaction with $n = 6$ (½ mark)
$E_{ce l l}^{\circ} = E_{c a t h o d e}^{\circ} - E_{an o d e}^{\circ}$ (½ mark)
$E_{ce l l}^{\circ} = 0.30$ V (1 mark)
Nernst equation applied to get 0.26 V (1 mark)

Revise the chapter

Chapter: Electrochemistry (notes, formulas, where marks are lost and every past question)
Topic: Nernst Equation, Gibbs Energy and Equilibrium Constant

Practise this chapterBoard-style questions from Electrochemistry, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/5/1: every question, by section.

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CBSE Class 12 Chemistry 2026: question 22

Hints

Step-by-step solution

Revise the chapter

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