5 Steps to a 5 AP Chemistry 2019

Equilibrium ❮ 225

There are two ways to solve this problem.

==

+

−

=×

+−

K xx −

x

[NH][OH]

[NH]

(3.00 )( )

(2.00 )

b^4 1.81^10

3

5

Assume x is small:

×=

=×

=

=−=

−

−

x

x

1.81 10

3.00

2.00

1.21 10

pOH 4.918

pH 14.000 4.918 9.082

5

5

Alternative solution:

=− ×+

=+

==

−

+

pOH log1.81 10 log

[NH]

[NH]

4.742 log

3.00

2.00

4.918 pH 9.082

(^54)
3
Titration Equilibria
An acid–base titration is a laboratory procedure commonly used to determine the concen-
tration of an unknown solution. A base solution of known concentration is added to an
acid solution of unknown concentration (or vice versa) until an acid–base indicator visually
signals that the endpoint of the titration has been reached. The equivalence point is the
point at which a stoichiometric amount of the base has been added to the acid. Both chemists
and chemistry students hope that the equivalence point and the endpoint are close together.
If the acid being titrated is a weak acid, then there are equilibria which will be established
and accounted for in the calculations. Typically, a plot of pH of the weak acid solution being
titrated versus the volume of the strong base added (the titrant) starts at a low pH and gradually
rises until close to the equivalence point, where the curve rises dramatically. After the equiva-
lence point region, the curve returns to a gradual increase. This is shown in Figure 15.3.
In many cases, one may know the initial concentration of the weak acid, but may be
interested in the pH changes during the titration. To study the changes one can divide the
titration curve into four distinctive areas in which the pH is calculated

1. Calculating the initial pH of the weak acid solution is accomplished by treating it as a
   simple weak acid solution of known concentration and Ka.


2. As base is added, a mixture of weak acid and conjugate base is formed. This is a buffer
   solution and can be treated as one in the calculations. Determine the moles of acid con-
   sumed from the moles of titrant added—that will be the moles of conjugate base formed.
   Then calculate the molar concentration of weak acid and conjugate base, taking into
   consideration the volume of titrant added. Finally, apply your buffer equations.


3. At the equivalence point, all the weak acid has been converted to its conjugate base. The
   conjugate base will react with water, so treat it as a weak base solution and calculate the
   [OH-] using Kb. Finally, calculate the pH of the solution.


4. After the equivalence point, you have primarily the excess strong base that will determine
   the pH.