Answer. Na2SO4 is formed by the reaction of strong base NaOH with strong acid H2SO4. It is formed by the complete replacement of ionizable H+ ions of H2SO4. As Na2SO4 does' not have replaceable H+ ions. It is a natural salt.
NaOH + H2SO4 Na2SO4 + H2O
While NaHS04 is formed by incomplete replacement of H+ off by a base:
H2S04 + NaOH ———> NaHSO4 + H2O
It further reacts with base to form normal salt:
NaHSO4(aq) + NaOH(aq) —————> Na2SO4(aq) + H2O
As NaHSO4 has one replaceable H+ ion. It can react with base so it is acidic salt.