The ionic product of water, Kw (strong acids and bases)

• How do we form the ionic product of water:

  Kw: [H+][OH-]

  Kw: 1 x 10^-14 at 298k (25 degrees celsius)

  
• How the Kw constant behaves in solution:

  • The value of Kw is fixed at a given temperature. So if the concentration of one thing in the equilibrium mixture changes then the concentrations of the others must change to keep the value of Kw the same.
  • Changing the temperature of the solution changes the value of Kw- the dissociation of water is an endothermic process, so warming the solution shifts the equilibrium to the right (more dissociation), and the value of Kw increases. Cooling the solution shifts the equilibrium to the right (less dissociation), and the value of Kw decreases.
• Finding the pH of water


  • In pure water, there is always one H+ ion for each OH- ion.
  • So [H+]=[OH-] = 1 x 10^-7
  • So Kw= [H+]^2
• Finding the pH of strong bases:

  • Strong bases donate one mole of OH- ions per mole of base.
  • This means that the concentration of OH- ions is the same as the concentration of the base.
  • However you need the [H+]

  1. Find the values of Kw and [OH-].
  2. Rearrange the equation, substitute the values for Kw and [OH-] into the equation, and solve it to find [H+].
  3. Substitute the [H+] into the pH equation.
• Finding the concentration of strong bases:


  1. Use the equation [H+] = 10^-pH to find the value of [H+] in the solution.
  2. Substitute your value for [H+] along with the value of Kw into the Kw equation and solve it to find [OH-].
  3. Because strong bases fully dissociate, [OH] is the same as the concentration of the base.
• How to find the pH of a mixture of 2 strong solutions:

  1. Find their moles using moles=concentration x volume.
  2. Calculate how many moles a solution is in excess and use this to calculate its concentration.
  3. This will give you either the [H+] or the [OH-], from there find the pH.