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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I

Q7(b). Aqueous LaCl₃ and LuCl₃: which is more acidic

Question

Consider aqueous solutions of LaCl₃ (lanthanum trichloride) and LuCl₃ (lutetium trichloride). Which solution shows lower pH? Explain.

Answer in brief

The LuCl₃ solution has the lower pH: Lu³⁺ is the smallest lanthanide ion (lanthanide contraction), so it polarises its coordinated water most strongly and is hydrolysed most.

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Topic notes: lanthanides and actinides (lanthanide contraction, magnetic moments, separation), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverslanthanide contractionhydrolysis of cationsLuCl3polarising powerbasicity of lanthanide hydroxides

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