16. Ions and Equilibrium;
Acids and Bases

The pH Scale
 Instead of saying that a corner block is one thousandth of the complete stack, or F = 0.001, we could say that relative to the stack as a whole, a single corner block represents pF = 3. This is the basis of the pH scale, with ions in solution replacing blocks in a stack. The pH of a solution is defined as the negative logarithm to base 10 of the hydrogen ion concentration: In the same way, pOH is the negative logarithm of the hydroxide ion concentration, pOH = -log10 [OH-], and pK is the negative logarithm of any equilibrium constant, pK = -log10 K. The table on page 7 gives the value of pKa as well as Ka. We can write the water dissociation equilibrium expression as Kw = [H+] [OH-] = 10-14; then pKw = pH + pOH = 14.00. The product of the concentrations of H+ and OH- ions is a constant, 10-14, and the sum of pH and pOH is another constant, 14. Pure water has a pH of 7.00, acids have pH values less than 7, and bases have pH values above 7. The pH of several commonly encountered liquids are shown below.
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