The H+ concentration of a solution is usually indicated in pH units on a pH scale that runs from 0 to 14. The pH value is equal to the logarithm of 1 over the H+ concentration:

where [H+] = molar H+ concentration. This can also be expressed as pH = -log [H+].

Common Acids and Bases





Hydrochloric acid


Sodium hydroxide


Phosphoric acid


Potassium hydroxide


Nitric acid


Calcium hydroxide


Sulfuric acid


Ammonium hydroxide


Carbonic acid


Pure water has a H+ concentration of 10-7 molar at 25° C, and thus has a pH of 7 (neutral). Because of the logarithmic relationship, a solution with 10 times the hydrogen ion concentration (10-6 M) has a pH of 6, whereas a solution with one-tenth the H+ concentration (10-8 M) has a pH of 8. The pH value is easier to write than the molar H+ concentration, but it is admittedly confusing because it is inversely related to the H+ concentration— that is, a solution with a higher H+ concentration has a lower pH value, and one with a lower H+ concentration has a higher pH value. A strong acid with a high H+ concentration of 10-2 molar, for example, has a pH of 2, whereas a solution with only 10-10 molar H+ has a pH of 10. Acidic solutions, therefore, have a pH of less than 7 (that of pure water), whereas basic (alkaline) solutions have a pH between 7 and 14 (table 2.3).

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