The GATE Grind

GATE 2025 ES – Question 37

Environmental Chemistry · Principles of Water Chemistry · 2 marks · Multiple choice

Label regions I–IV of the phosphoric-acid speciation diagram.

Diagram for GATE 2025 ES question 37
  1. $PO_4^{3-},HPO_4^{2-},H_2PO_4^-,H_3PO_4$
  2. $H_3PO_4,HPO_4^{2-},H_2PO_4^-,PO_4^{3-}$
  3. $H_3PO_4,H_2PO_4^-,HPO_4^{2-},PO_4^{3-}$
  4. $PO_4^{3-},H_2PO_4^-,H_3PO_4,HPO_4^{2-}$

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (C) $H_3PO_4,H_2PO_4^-,HPO_4^{2-},PO_4^{3-}$

Explanation

Increasing pH successively removes protons from phosphoric acid: $H_3PO_4\to H_2PO_4^-\to HPO_4^{2-}\to PO_4^{3-}$. The low-pH curve I is fully protonated; the intermediate dominance curves II and III lose one and two protons; high-pH IV is phosphate. The order is **C**.

More Principles of Water Chemistry questions

All Principles of Water Chemistry questions →