5 Steps to a 5 AP Chemistry 2019

Reactions and Periodicity ‹ 81

For example, suppose you wanted to determine the molarity of an HCl solution.
You would pipet a known volume of the acid into a flask and add a couple of drops of a
suitable acid–base indicator. An indicator that is commonly used is phenolphthalein, which
is colorless in an acidic solution and pink in a basic solution. You would then fill a buret
with a strong base solution (NaOH is commonly used) of known concentration. The buret
allows you to add small amounts of the base solution to the acid solution in the flask. The
course of the titration can also be followed by the use of a pH meter. Initially the pH of the
solution will be low, since it is an acid solution. As the base is added and neutralization of
the acid takes place, the pH will slowly rise. Small amounts of the base are added until one
reaches the equivalence point. The equivalence point is that point in the titration where
the number of moles of H+ in the acid solution has been exactly neutralized with the same
number of moles of OH-:

molesH+−=molesOHattheequivalencepoint

For the titration of a strong acid with a strong base, the pH rapidly rises in the vicin-
ity of the equivalence point. Then, as the tiniest amount of base is added in excess, the
indicator turns pink. This is called the endpoint of the titration. In an accurate titration,
the endpoint will be as close to the equivalence point as possible. For simple titrations that
do not use a pH meter, it is assumed that the endpoint and the equivalence point are the
same, so that:

molesH+−=molesOHattheendpoint

After the equivalence point has been passed, the pH is greater than 7 (basic solution)
and begins to level out somewhat. Figure 6.1 shows the shape of the curve for this titration.
Reaction stoichiometry can then be used to solve for the molarity of the acid solution.
See Chapter 7, Stoichiometry, for a discussion of solution stoichiometry.
An unknown base can be titrated with an acid solution of known concentration. One
major difference is that the pH will be greater than 7 initially and will decrease as the titra-
tion proceeds. The other major difference is that the indicator will start pink, and the color
will vanish at the endpoint.

Figure 6.1 Titration of a strong acid with a strong base.

pH7.0

Vol NaOH added (mL)

Equivalence

point