NEET UG 2023 — Chemistry Physical Chemistry

2023mcqmedium

On balancing the given redox reaction

$a{\mathrm{Cr}}_{2}{O}_{7}^{2-}+{\mathrm{bSO}}_{3}^{2-}(\mathrm{aq})+{\mathrm{cH}}^{+}(\mathrm{aq})\overset{}{\rightarrow }2{\mathrm{aCr}}^{3+}(\mathrm{aq})+{\mathrm{bSO}}_{4}^{2-}(\mathrm{aq})+\frac{c}{2}{H}_{2}O(l)$

The coefficients $a,b,c$ are found to be respectively

Official previous-year question

Held on 30 Apr 2023 · Verified 9 Jul 2026.

Options

A
$8,1,3$
B
$1,3,8$
C
$3,8,1$
D
$1,8,3$

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Solution

Reduction Half reaction : ${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+6{e}^{-}\rightarrow 2{\mathrm{Cr}}^{3+}$

Oxidation Half reaction :

${{\mathrm{SO}}_{3}}^{2-}\rightarrow {{\mathrm{SO}}_{4}}^{2-}+2{e}^{-}\times 3$

Overall reaction :

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}$

To balance $‘O’$ atoms, adding ${H}_{2}O$ on LHS

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}+4{H}_{2}O$

To balance $‘H’$ atoms, adding ${H}^{+}$ on RHS

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}+8{H}^{+}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}+4{H}_{2}O$

Therefore, $a=1,b=3,c=8$ .

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Solution

Reduction Half reaction : ${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+6{e}^{-}\rightarrow 2{\mathrm{Cr}}^{3+}$

Oxidation Half reaction :

${{\mathrm{SO}}_{3}}^{2-}\rightarrow {{\mathrm{SO}}_{4}}^{2-}+2{e}^{-}\times 3$

Overall reaction :

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}$

To balance $‘O’$ atoms, adding ${H}_{2}O$ on LHS

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}+4{H}_{2}O$

To balance $‘H’$ atoms, adding ${H}^{+}$ on RHS

${\mathrm{Cr}}_{2}{{O}_{7}}^{2-}+3{{\mathrm{SO}}_{3}}^{2-}+8{H}^{+}\rightarrow 2{\mathrm{Cr}}^{3+}+3{{\mathrm{SO}}_{4}}^{2-}+4{H}_{2}O$

Therefore, $a=1,b=3,c=8$ .

NEET UG 2023 — Chemistry Physical Chemistry

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NEET UG 2023 — Chemistry Physical Chemistry

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