Pure water contains an equal concentration of hydrogen and hydroxide ion.

Thus, it is a neutral substance, neither acidic nor basic. If a solution has an excess of

hydrogen ions, the solution will be acidic. If a solution has an excess of hydroxide ions,

the solution will be alkaline. It is customary to use the hydrogen ion concentration (pH)

as a measure of the acidity or alkalinity of a solution. pH is a unit of concentration that

allows the technician to work with very dilute concentrations of hydrogen and hydroxide

ion in a convenient form. This scale permits the representation of the enormous range

+

+

+

of the possible [H ] concentrations from a 1.0 mol/L [H ] to a 1.0 X 10-14 mol/L [H ].

The notation p denotes "negative log of." Logarithms are discussed in lesson 1,

and there is a four-place logarithm table in Appendix B. pH is mathematically defined

as:

+

+

-log [H ] or log 1/[H ]

a. **Example 1. **What is the pH of a 0.133 mol/L HCl solution?

Solution. Select the expression that allows you to solve for the unknown

quantity.

+

pH = -log [H ]

Substitute the given information, and solve for the unknown quantity.

pH = -log 0.133 = --log [1.33 X 10--1] = --(0.1239 -- 1)

pH = -(-0.876)

pH = 0.876

b. **Example 2**. What is the pH of a 0.020 mol/L acid solution that is ionized 1.8

percent?

Solution. From previous discussion, hydrogen or hydroxide does not

contribute to the acidity or alkalinity of a solution unless ionized. So in this problem

consider only that hydrogen ion that is ionized.

+

pH = -log [H ]

pH = -log [(0.020 mol/L) (0.018)] = --log [3.6 x 10--4] = --(.5563 --4)

pH = 3.4

MD0837

9-9

Integrated Publishing, Inc. |