It is number of H+ ions furnished by a molecule of an acid. An acid may be classified according to its basicity. Thus we may have,
(i) Mono basic or Mono protic acids like HCl, HNO3, CH3COOH, HCN etc.
(ii) Dibasic or Diprotic acids like, H2SO4, H2CO3, H2SO3, H2S etc.
(iii) Tribasic or Triprotic acids like H3PO4 ,H3AsO4 etc.
