pH of water

• An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions

H₂O(l) ? H⁺(aq) + OH^-(aq)

• The equilibrium constant for this reaction is:

K_c?x [H₂O] = [H⁺] [OH^-]

• Since the concentration the H⁺?and OH^-?ions is very small, the concentration of water is considered to be a constant, such that the expression can be rewritten as:

K_w?= [H⁺] [OH^-]

Where?K_w?(ionic product of water)=K_c?x [H₂O]

?=10^-14?mol^2?dm^-6?at 298K

• The product of the two ion concentrations is always?10^-14?mol^2?dm^-6
• This makes it straightforward to see the relationship between the two concentrations and the nature of the solution:

[H⁺] & [OH^-] Table

Answer:

The correct option is?D.

• • Since?K_w?= [H⁺] [OH^-] , rearranging gives [H⁺] ?=?K_w?÷ [OH^-]
  • The concentration of ?[H⁺] is (1.0 × 10^?14) ÷ (1.0 × 10^?3) = 1.0 × 10^?11?mol dm^?3
  • So the?pH = 11