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Highly selective Backlog Qs for JEE MainChemistryIonic Equilibrium

A weak acid of dissociation constant 10⁻⁵ is being titrated with aqueous NaOH solution. The pH at the point of one-third neutralisation of the acid will be

Options

  1. A5+ 2- 3
  2. B5- 2
  3. C5- 3
  4. D5- 6

Correct answer

B. 5- 2

Step-by-step solution

Hints: K _ a =10⁻⁵ pK _ a =- K _ a =- 10⁻⁵=5 array llcc & HA + NaOH NaA + H ₂ O Initial & 1 ~mole 0 & 0 & 0 Final & (1-1 / 3) mole & 1 / 3 ~mole & 1 / 3 ~mole & =2 / 3 ~mole & 1 / 3 ~mole array (Assumed weak acid to be monoprotic, since only one dissociation constant value is provided) Final solution acts as an acidic buffer. pH = pK _ a + [ salt ] [ Acid ] pH =5+ 1 3 2 3 =5+ 1 2 pH =5- 2

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