pH and Kw

 

Water will self ionise according to the equation below.

H2O(l) + H2O(l) <=> H3O+(aq) + OH-(aq)

Because of self ionisation water will conduct electricity ever so slightly.

The equilibrium expression for this equation is given below.

But since the concentration of water molecules in an aqueous solution is virtually constant we can write the expression as


In pure water, at 25oC, the [H3O+] = [OH-] = 10-7
Therefore Kw = 10-14
The concentration of hydronium and hydroxide ions is very small and this is why pure water will conduct electricity to a small extent. This relationship allows us to calculate the concentration of hydronium and hydroxide ions in solution at 25oC.

Knowing the hydroxide concentration of a solution we are able to find the pH via the expression above. We can change the expression to solve for the concentration of hydronium ions

Or we can change the expression to solve for the concentration of hydroxide ions.


Example. What is the pH of a 0.1 M NaOH solution?




Example What is the [OH-] of a solution with pH 8?

Step 1 find the concentration of hydronium ions



Step 2 using the expression below find the [OH-]



Exercises

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