Question

The dihydrogen phosphate ion is an amphoteric substance. Its Ka is 6.3x10^-8. When sodium dihydrogen phosphate is dissolved in water, will the ion act as an acid or as a base? Explain why.

Answer 1

NaH2PO4 (aq) ---------> HPO4 2- + H+

for the above reaction Ka = [HPO4 2-] [H+] / [NaH2PO4] = 6.3 * 10^-8

Ka is directly proportional to product concentration and inversely propotional to reactants concentration. and Ka is very weak which indicate that product formation is not favourable. so the species do not acts as acid.

H2PO4 2- + H+ ------> H3PO4

for this reaction Ka will become 1 / (6.3 *10^-8) = 1.587 * 10^7 which implys that forward reaction is favourable. Hence dihydrogen phosphate ion can accept H+ ion easily so it acts as a base.