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Consider the following three reactions involving dihydrogen phosphate ion: (i) H3PO4 + H2O → H3O+ + H2PO4− (ii) H2PO4− + H2O → HPO4²− + H3O+ (iii) H2PO4− + OH− → H3PO4 + O2− In which of these reactions does H2PO4− behave as an acid?

1. (ii) only
2. (i) and (ii)
3. (iii) only
4. (i) only

Correct answer: (ii) only

Solution

In (ii) H2PO4- -> HPO4^2- + H3O+, the dihydrogen phosphate donates a proton, so it acts as an acid. In (i) it is a product and in (iii) it gains H+ (acts as a base). Hence only (ii).

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