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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3el054

A litre of hard water contains of . Taking the scum as calcium stearate, , select all correct statements.

  1. AAbout of sodium stearate is wasted as scum before cleaning can begin.
  2. BThe scum is calcium stearate.
  3. CSodium lauryl sulphate in this water would also give a precipitate of calcium lauryl sulphate.
  4. DSoap lost as scum cannot form micelles to clean.

Answer: A, B, D

Each Ca2+ takes two stearate ions: 2 x 0.0040 = 0.0080 mol (a true), as insoluble calcium stearate (b true). Calcium lauryl sulphate is soluble, so no precipitate forms (c false). Stearate bound in the precipitate is not available for micelle formation (d true).
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