Three reactions involving H ₂ PO ₄⁻ are given below : (i) H ₃ PO ₄+ H ₂ O H ₃ O ⁺+ H ₂ PO ₄⁻ (ii) H ₂ PO ₄⁻+ H ₂ O HPO ₄²⁻+ H ₃ O ⁺ (iii) H ₂ PO ₄⁻+ OH ⁻ H ₃ PO ₄+ O ²⁻ In which of the above does H ₂ PO ₄⁻ act as an acid ?

Step-by-step solution

(i) H ₃ PO ₄+ H ₂ H ₃ O ⁺+ acriugate base H ₂ PO ₄⁻ (ii) H ₂ PO ₄⁻+ H ₂ O aconiugate base HPO ₄⁻² + H ₃ O ⁺ (iii) H ₂ PO ₄⁻+ acid OH ⁻ conjugate acid H ₃ PO ₄ + O ⁻² Only in reaction (ii) H ₂ PO ₄⁻ acids as 'acid'.