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Water dissociation constant (K _w)

• K_w represents the dissociation constant for water.
  
  
• It is similar to the acid dissociation constant (K_a).
  
  
• Water (H₂O) partially dissociates into hydrogen ions (H⁺) and hydroxide ions (OH^-) in
  equal concentrations of 10 ^{- 7}M.
  
  
  H₂O < = > H⁺ + OH^-
  
  
  The extent of this dissociation, at equilibrium, is represented by the water dissociation constant (K_w) -
  
  
  [H⁺] [OH^-]…………..(product of the molar concentration (M) of the dissociated forms)
  K_w = ---------------……………(divided by)
  [H₂O]………………..(the molar concentration (M) of the undisociated form)
  
  
  
  10 ^{- 7} x 10 ^{- 7}
  = ..… ------------------ ….M …… (water has a one molar concentration by definition)
  1
  
  
  
  Therefore, K_w equals 10 ^{- 14}