Ion-product constant for water

The ion-product constant for water describes the relationship between molecular water and its dissociated ionic components at dynamic equilibrium.

Pure water dissociates partially to give the hydroxonium and hydroxide ions.

2H_2O(l)\rightleftharpoons H_3O^+(aq)+OH^-(aq)

Since water is the solvent, its activity approximately equals to one and the equilibrium constant is

K_w=\left [ H_3O^+ \right ]\left [ OH^- \right ]\; \; \; \; \; \; \; \; 11

Kw is called the ion-product constant for water. Conductivity measurements of pure water reveals that [H3O+] = 10-7 M at 25oC. Since the concentration of hydroxonium ions and hydroxide ions are formed only from the dissociated of water, [H3O+] = [OH], giving Kw = 10-14 M at 25oC . It is important to note that Kw remains equal to 10-14 M at 25oC, regardless of whether the H3O+ and OHions originate from the dissociation of water or from added acids or bases.

 

Question

What is the concentration of OH when HCl is added to water at 25oC to give a pH of 2?

Answer

According to Le Chatelier’s principle, the increased [H3O+] shifts the position of the equilibrium of eq11 to the left to attain a new equilibrium where

[OH^-]=\frac{10^{-14}}{10^{-2}}=10^{-12}\: M

 

 

Next article: Acid/base dissociation constant
Previous article: Why is water sometimes omitted from the equilibrium constant?
Content page of chemical equilibrium
Content page of intermediate chemistry
Main content page

Leave a Reply

Your email address will not be published. Required fields are marked *

Mono Quiz