The Foundations of Chemistry

(Marcin) #1
EXAMPLE 18-6 Calculations Involving pH and pOH
Calculate [H 3 O], pH, [OH], and pOH for a 0.015 MCa(OH) 2 solution.
Plan
We write the equation for the ionization of the strong base Ca(OH) 2 , which gives us [OH].
Then we calculate pOH. We use the relationships [H 3 O][OH]1.0^10 ^14 and
pHpOH14.00 to find pH and [H 3 O].
Solution
H 2 O
Ca(OH) 2 8888nCa^2 2OH 

Because calcium hydroxide is a strong base (it dissociates completely), we know that

[OH] 2 0.015 M 0.030 M

pOHlog [OH]log (0.030)(1.52) 1.52

We also know that pHpOH14.00. Therefore,

pH14.00pOH14.001.52 12.48

Because [H 3 O][OH]1.0 10 ^14 , [H 3 O] is easily calculated.

[H 3 O]3.3 10 ^13 M

You should now work Exercises 28 and 29.

The pH of a solution can be determined using a pH meter (Figure 18-1) or by the
indicator method. Acid–base indicatorsare intensely colored complex organic compounds
that have different colors in solutions of different pH (Section 19-4). Many are weak acids
or weak bases that are useful over rather narrow ranges of pH values. Universal indicators
are mixtures of several indicators; they show several color changes over a wide range of
pH values.

1.0 10 ^14

0.030

1.0 10 ^14

[OH]

A pH meter is much more accurate
than an indicator for obtaining pH
values.


760 CHAPTER 18: Ionic Equilibria I: Acids and Bases


Figure 18-1 A pH meter gives the pH of the
solution directly. When the electrode is dipped
into a solution, the meter displays the pH. The
pH meter is based on the glass electrode. This
sensing device generates a voltage that is
proportional to the pH of the solution in which
the electrode is placed. The instrument has an
electrical circuit to amplify the voltage from the
electrode and a meter that relates the voltage to
the pH of the solution. Before being used, a pH
meter must be calibrated with a series of
solutions of known pH.
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