In electrolysis of NaCl, when Pt electrode is taken, then H₂ is liberated at cathode while with Hg cathode, it forms sodium amalgam. This is because

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Conc. of H⁺ ions is larger, when Pt electrode is taken

Hg is more inert than Pt

Na is dissolved in Hg, while it does not dissolve in Pt

More voltage is required to reduce H⁺ at Hg than at Pt

answer is B.

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Detailed Solution

Sodium chloride in water dissociates as
NaCl ⇌ Na⁺+ Cl−
H₂O ⇌ H⁺+ OH⁻
When electric current is passed through this solution using platinum electrodes, Na⁺and H⁺ move towards cathode whereas Cl⁻and OH⁻ ions move towards anode.
At cathode:
H⁺+ e⁻→ H
H + H → H₂
At anode
Cl⁻→ Cl + e⁻
Cl + Cl → Cl₂
If mercury is used as cathode, H⁺ ions are not discharged at mercury cathode because mercury has a high hydrogen over voltage.
Na⁺ ions are discharged at cathode in preference of H⁺ ions yielding sodium, which dissolves in mercury to form sodium amalgam.