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GATE 2021 CE (CE1) – Question 44

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 2 marks · Multiple choice

Wastewater ammonia nitrogen exists as NH₄⁺ and NH₃. Considering pH alone, which statement is correct?

  1. Above pH 9.25 only NH₄⁺ is present.
  2. Below pH 9.25 NH₃ predominates.
  3. At pH 7 the two species are equal.
  4. At pH 7 NH₄⁺ predominates.

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Show answer and explanation

Correct answer: (D) At pH 7 NH₄⁺ predominates.

Explanation

The equilibrium has pKₐ≈9.25 and $[NH_3]/[NH_4^+]=10^{pH-pK_a}$. At pH 7 this ratio is about 0.00562, so ammonium strongly predominates. Equal amounts occur near pH 9.25, and higher pH favours NH₃. **Answer: D**.