section_path: Acid–base equilibrium
section_name: Acid–base equilibrium
paragraph_text: Acid–base equilibrium
  
  The reaction of a strong acid with a strong base is essentially a quantitative reaction. For example, HCl (aq) + Na (OH) (aq) ⟶ H 2 O + NaCl (aq) {\displaystyle {\ce {HCl_{(aq)}{}+ Na(OH)_{(aq)}-> H2O + NaCl_{(aq)}}}}
  
  In this reaction both the sodium and chloride ions are spectators as the neutralization reaction, H + OH − ⟶ H 2 O {\displaystyle {\ce {H + OH- -> H2O}}} does not involve them. With weak bases addition of acid is not quantitative because a solution of a weak base is a buffer solution. A solution of a weak acid is also a buffer solution. When a weak acid reacts with a weak base an equilibrium mixture is produced. For example, adenine, written as AH, can react with a hydrogen phosphate ion, HPO 2− 4. AH + HPO 4 2 − ↽ − − ⇀ A − + H 2 PO 4 − {\displaystyle {\ce {AH + HPO4^2- <=> A- + H2PO4-}}}
  
  The equilibrium constant for this reaction can be derived from the acid dissociation constants of adenine and of the dihydrogen phosphate ion. = K a 1 = K a 2 {\displaystyle {\begin{aligned}\left\!\left&=K_{a1}{\bigl }\\\left\!\left&=K_{a2}\left\end{aligned}}}
  
  The notation signifies "concentration of X". When these two equations are combined by eliminating the hydrogen ion concentration, an expression for the equilibrium constant, K is obtained. = K; K = K a 1 K a 2 {\displaystyle \left\!\left=K{\bigl }\!\left;\quad K={\frac {K_{a1}}{K_{a2}}}}