pH, pOH, and K w

As we explain in Chapter 16, pH and pOH are measurements of the acidity or basicity of
aqueous solutions:
Pure water spontaneously self-ionizes to a small degree, leading to equal concentrations of
hydrogen and hydroxide ions ( , or ):

.) The product of these two concentrations is the ion-product
constant for water, K
W
:
As a result, the pH and the pOH of pure water both equal 7. Acidic solutions have pH values
less than 7 and pOH values greater than 7. Basic (alkaline) solutions have pH values greater
than 7 and pOH values less than 7.
When you add an acid or base to an aqueous solution, the concentrations of hydrogen and
hydroxide ions shift in proportion so that the following is always true:

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