Ionic Product of Water

Physics Chemistry  Biology  Mathematics
Science > Chemistry > Ionic EquilibriaYou are Here

Ion Equilibrium in Water and Ionic Product of Water:

  • Water has electrical conductivity, hence it must undergo dissociation. “Dissociation of pure water to a very little extent into H+ and OH ions by  itself is called as self ionisation of water. Water is very weak electrolyte. In water, an equilibrium between ions and unionised water molecule exists as,

H2O     ⇌        H+  +    OH

H+  +  H2O    ⇌ H3O+

The net reaction is

H2O  +  H2O    ⇌   H3O+ +    OH

H3O+ is called hydronium ion

Applying law of mass action to above equilibrium, we have

  • Now water is very weak electrolyte. It dissociates in very small amount. Hence practically the concentration of unionised water is almost same as starting concentration. Hence [H2O] = constant. Similarly Practically [H3O+] = [H+]. Therefore the equation (1) becomes.

This relation is known as ionic product of water.

  • The product of molar concentration of H+ and OH- ions pure water or any aqueous solution at constant temperature is constant which is called as ionic product of water.
  • At 298 K, for pure water [H+] = [OH] = 1 x 10-7 mole dm-3

Thus Kw = [H+] [OH] = 1 x 10-7)(1 x 10-7) = 1 x 10-14

Thus at 298 K ionic product of water is 1 x 10-14

  • When small amount of acid is added to water, the concentration of H+ ions increases and that of OH ions decrease. i.e. [H+] > [OH] i.e. [ H+]    > 1 x 10-7
  • When an alkali is added to water then OH ion concentration becomes higher than that of H+ ions. i.e. [OH] > [H+] i.e.[OH]. > 1 x 10-7
  • In neutral solution. H+ and OH ion concentration are equal. i.e. [H+] = [OH] = 1 x 10-7 mole dm-3
  • Thus concept of ionic products of water helps us in classifying aqueous solutions as acidic, basic and neutral.

pH and pOH of a solution:

pH of a solution:

  • The negative logarithm to the base 10 of molar concentration of H+ ions in a solution is called as pH of a solution.

Mathematically, pH = – log10[H+]

For pure water or a neutral solution.at 298  K.

[H+] = 1 x 10-7 moles/dm3

∴ pH = – log10(1 x 10-7)  = – (-7) log1010 = + 7 (1) = 7

pOH of a Solution:

  • The negative logarithm to the base 10 of molar concentration of OH ions in a solution is called as pOH of a solution.

Mathematically, pOH = – log10[OH]

For pure water or a neutral solution.at 298  K.

[OH] = 1 x 10-7 moles/dm3

∴ pOH = – log10(1 x 10-7)  = – (-7) log1010 = + 7 (1) = 7

Relation Between pH  and pOH:

Ionic product of water is given by Kw = [H+] [OH]

For pure water or an aqueous solution, Kw = 1 x 10-14   at 298   K

 ∴  [H+] [OH]  = 1 x 10-14

Taking logs of both sides of the equation to the base 10

 log10[H+] + log10[OH] = log10(1 x 10-14)

 ∴ log10[H+] + log10[OH] = -14 log1010

 ∴ log10[H+] + log10[OH] = -14(1) = -14

Multiplying both sides of the equation by -1

∴  – log10[H+] – log10[OH] =  14

But  pH = – log10[H+] and  pOH = – log10[OH]

∴ pH  + pOH  = 14

Thus the sum of pH  and  pOH for pure water or any aqueous solution is equal to 14.

pH Scale or Sorensen’s Scale:

  • Scientist Sorensen in 1909 introduced a convenient scale to express hydrogen ion  (H+) concentration to decide acidic, alkaline or neutral nature of the solution , is known as pH scale.
  • The negative logarithm to the base 10 of the molar concentration of H+ ions in a solution is called as pH of a solution. The pH scale expresses all degrees of acidity or alkalinity of a dilute aqueous solution.
  • As the concentration of acid decreases the pH value increases from 0 to7.while as the concentration of base decreases the pH value decreases from 14 to7.
  • For pure water or aqueous neutral solution,  pH = 7.
  • It is to be noted that pH scale is used for a dilute aqueous solution only. i.e. their molarity is less than 1 M.

Two acids monobasic and diabasic have the same pH. Does this mean that the molar concentration of the two acids is identical?

A monobasic acid dissociates as
HA ⇌ H+ + A
Thus 1 mole of monobasic acid gives 1 mole of H+ ions.
A dibasic acid dissociates as
H2A ⇌ 2H+ + A
Thus 1 mole of dibasic acid gives 2 moles of H+ ions.

  • Hence if the pH of the two solutions is equal, the molar concentration of monobasic acid will be twice of the molar concentration of dibasic acid.
Science > Chemistry > Ionic EquilibriaYou are Here
Physics Chemistry  Biology  Mathematics

Leave a Comment

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