Chapter 9: Unit 6. Dissociation of Water
Dissociation of Water
Water is amphoteric compound which can behave as an acid or as a base depending on the condition it is in. The dissociation of water is shown below where one water molecule acts as an acid and the other molecule acts as a base:
Acid Base Conjugate AcidConjugate Base
The dissociation of water is also called Auto–Ionization Water or Self–Ionization of Water.
H2O(l) + H2O(l) ⇔H3O + (aq) + OH–(aq)
The dissociation constant of water is called Water Dissociation Constant
Water Dissociation Constant Expression:
K = {[ H3O + (aq)] x [OH–(aq)]} / {[ H2O(l)] x [H2O(l)]}
H O(l) is pure liquid water and its concentration is not changed. Therefore, H 2 O(l) will be discarded from the expression and K will be replaced by K w :
Kw= {[ H3O + (aq)] x [OH–(aq)]}
It was determined by experiment at 25oC that [H3O + (aq)] = [OH–(aq)] = 1.0 x 10–7 mol/L
The value of K w = [1.0 x 10–7 ] x [1.0 x 10–7 ] = 1.0 x 10–14 at 25 o C
According to the concentrations of [H 3 O + (aq)] and [OH–(aq)], one can determine the acidity and the basicity of a solution. For example:
If [H3O + (aq)] is greater than [OH–(aq)], then the solution is said to be acidic.
If [H3O + (aq)] is equal [OH–(aq)], then the solution is said to be neutral.
If [OH–(aq)] is greater than [H3O + (aq)], then the solution is said to be basic.
Calculations Involving Kw , [H3O + ] and [OH–]
The video link below is illustrating the concept of water dissociation:
https://study.com/academy/lesson/dissociation–constant–and–autoionization–of–water.html
Another Video You Tube illustrates the calculations of [H3O + ] and [OH–] using Kw
[H3O + ] or [OH–] can be determined if one or the other is known using K w .
Examples:
At room temperature 25oC, a solution has the [H3O + ] = 2.5 x 10–3 M. Find the [OH–] of this solution.
Solution:
Using the formula below:
K w= {[ H3O + (aq)] x [OH–(aq)]} = 1.0 x 10–14 at 25 o C
[OH–] = [1.0 x 10–14 ] / [[H 3 O + ] = [1.0 x 10–14 ] / [2.5 x 10–3 ] = 4.0 x 10–12