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222 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
15-4 pH
Quick Tip
The concentration of water is a constant and is incorporated into the K w.
The numerical value for the K w of 1.00 10 14 is for the product of the [H ]
and [OH ] in pure water and for all aqueous solutions.
In an aqueous solution, there are two sources of H , the acid and water.
However, the amount of H that the water dissociation contributes is very small
and is normally easily ignored.
Because the concentration of the hydronium ion, H 3O (or H as a shorthand
notation), can vary tremendously in solutions of acids and bases, a scale to represent
the acidity of a solution was developed. This is the pH scale, which
relates to the [H 3O ]:
pH log [H 3O ] or log [H ] using the shorthand notation.
Remember that in pure water K w [H 3O ] [OH ] 1.00 10 14 . Since both
the hydronium ion and hydroxide ions form in equal amounts, x, the K w expression
is:
[x] 2 1.00 10 14
The value of K w only applies at or near room temperature. This value, like all equilibrium
constants will vary with temperature.
Solving for [x] gives us x [H 3O ] 1.00 10 7 . If you then calculate the pH
of pure water:
pH log [H 3O ] log [1.00 10 7 ] (7.00) 7.00