The Foundations of Chemistry

The reaction produces 100. mmol of potassium sulfate. This is contained in 300. mL of solu-
tion, and so the concentration is

_?_ 0.333 MK 2 SO 4

You should now work Exercises 10 and 14.

TITRATIONS

In Examples 3-23 and 11-3, we calculated the volume of one solution that is required to
react with a given volume of another solution, with the concentrations of bothsolutions
given. In the laboratory we often measure the volume of one solution that is required to
react with a given volume of another solution of known concentration. Then we calcu-
late the concentration of the first solution. The process is called titration(Figure 11-1).

11-2

100. mmol K 2 SO 4
     


101. mL

mmol K 2 SO 4



mL

Because K 2 SO 4 is a strong electrolyte,

this corresponds to 0.666 MK
and

0.333 MSO 42 .

11-2 Titrations 405

Figure 11-1 The titration process. (a) A typical setup for titration in a teaching laboratory.
The solution to be titrated is placed in an Erlenmeyer flask, and a few drops of indicator are
added. The buret is filled with a standard solution (or the solution to be standardized). The
volume of solution in the buret is read carefully. (b) The meniscus describes the surface
of the liquid in the buret. Aqueous solutions wet glass, so the meniscus of an aqueous
solution is always concave. The position of the bottomof the meniscus is read and recorded.
(c) The solution in the buret is added (dropwise near the end point), with stirring, to the
Erlenmeyer flask until the end point is reached. (d) The end point is signaled by the
appearance (or change) of color throughoutthe solution being titrated. (A very large excess of
indicator was used to make this photograph.) The volume of the liquid is read again — the
difference between the final and initial buret readings is the volume of the solution used.

(a) (b) (c) (d)