The Foundations of Chemistry

Titration is the process in which a solution of one reactant, the titrant, is carefully

added to a solution of another reactant, and the volume of titrant required for

complete reaction is measured.

How does one know when to stop a titration — that is, when is the chemical reaction

just complete? In one method, a few drops of an indicatorsolution are added to the solu-

tion to be titrated. An indicatoris a substance that can exist in different forms, with

different colors that depend on the concentration of H
in the solution. At least one of

these forms must be very intensely colored so that even very small amounts of it can be

seen.

We can titrate an acid solution of unknown concentration by adding a standardized

solution of sodium hydroxide dropwise from a buret(see Figure 11-1). A common buret

is graduated in large intervals of 1 mL and in smaller intervals of 0.1 mL so that it is

possible to estimate the volume of a solution dispensed to within at least0.02 mL.

(Experienced individuals can often read a buret to0.01 mL.) The analyst tries to choose

an indicator that changes color clearly at the point at which stoichiometrically equivalent

amounts of acid and base have reacted, the equivalence point.The point at which the

indicator changes color and the titration is stopped is called the end point.Ideally, the

end point should coincide with the equivalence point. Phenolphthalein is colorless in acidic

solution and reddish violet in basic solution. In a titration in which a base is added to an

acid, phenolphthalein is often used as an indicator. The end point is signaled by the first

appearance of a faint pink coloration that persists for at least 15 seconds as the solution

is swirled.

EXAMPLE 11-5 Titration

What is the molarity of a hydrochloric acid solution if 36.7 mL of the HCl solution is required

to react with 43.2 mL of 0.236 Msodium hydroxide solution?

HCl
NaOH88nNaCl
H 2 O

Plan

The balanced equation tells us that the reaction ratio is one millimole of HCl to one millimole

of NaOH, which gives the unit factor, 1 mmol HCl/1 mmol NaOH.

HCl 
NaOH88n NaCl
 H 2 O

1 mmol 1 mmol 1 mmol 1 mmol

First we find the number of millimoles of NaOH. The reaction ratio is one millimole of HCl

to one millimole of NaOH, so the HCl solution must contain the same number of millimoles

of HCl. Then we can calculate the molarity of the HCl solution because we know its volume.

Solution

The volume of a solution (in milliliters) multiplied by its molarity gives the number of

millimoles of solute.

_?_ mmol NaOH43.2 mL NaOH soln10.2 mmol NaOH

Because the reaction ratio is one millimole of NaOH to one millimole of HCl, the HCl solu-

tion must contain 10.2 millimoles of HCl.

0.236 mmol NaOH



1 mL NaOH soln

The choice of indicators will be
discussed in Section 19.4.

406 CHAPTER 11: Reactions in Aqueous Solutions II: Calculations

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 5-15, Titration Simulation.