Self ionization of water- Kw


Water is an amphiprotic molecule.This ability helps water to act as both a proton donor and a proton acceptor, this property of water suggests that a proton transfer exists  for pure water

2 H2O (l) <-------------------> OH- (aq)  + H3O+ (aq)

the equilibrium constant for this reaction is water equilibrium constant (Kw).Its value is 1.0 x 10-14 at 25 c.

In pure water the hydronium and hydroxide ion concentrations are equal:We find these concentrations by taking the square root of Kw :

[H3O+] = [OH- ] = 1.0 x 10-7 M

Equal concentration of these two ions means that pure water is neither acidic nor basic.