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Highly selective Backlog Qs for JEE MainChemistryIonic Equilibrium

K a for the reaction, Fe 3 + ( aq ) + 2H 2 O ( l ) ⇌ Fe ( OH ) 2 + ( aq ) + H 3 O + is   6.5 × 10 - 4 . What is the maximum pH value, which could be used, so that at least 80 % of the total iron III in a dilute solution exist as Fe 3 + ?

Options

  1. A4 . 60
  2. B2 . 59
  3. C7 . 59
  4. D7 . 0

Correct answer

B. 2 . 59

Step-by-step solution

This problem is based on the conceptual mixing of equilibrium constant and pH of the solution and given that, Fe 3 + aq + 2 H 2 O l ⇌ Fe ( OH ) 2 + aq + H 3 O + aq K a = 6 . 5 × 10 - 4 and 80 % of Fe 3 + present in the final solution means only 20 % is dissociated. So, Fe 3 +   = 0 . 8   Molar   and   Fe OH 2 + = 0 . 2   Molar Now, K a = Fe OH 2 + H 3 O + Fe 3 + 6.5 × 1 0 - 4 = 0.20 0.80 H 3 O + H 3 O + = 6.5 × 1 0 - 4 × 4 H 3 O + = 26.0 × 1 0 - 4 = 2.6 ×

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