PH

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

Acid

Symbol

Base

Symbol

Hydrochloric acid

HCl

Sodium hydroxide

NaOH

Phosphoric acid

H3PO4

Potassium hydroxide

KOH

Nitric acid

HNO3

Calcium hydroxide

Ca(OH)2

Sulfuric acid

H2SO4

Ammonium hydroxide

NH4OH

Carbonic acid

H2CO3

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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