Question: why nah2po4 is called an acid salt?
Answer: Acids salts contain acidic or replaceable hydrogen in their composition. They react with further amounts of bases to give normal salts (NaHSO4 + NaOH = Na2SO4 + H2O). An 'acid salt' need not be acidic in nature. ... Thus, NaHSO4 and NaH2PO4 are acidic whereas NaHCO3 and Na2HPO4 are distinctly basic in aqueous medium.