Buffers and Equilibrium
A mixture of aqueous solutions of a weak acid and its conjugate base will act as a buffer and prevent pH
change. Consider the phosphate-biphosphate buffer system present in the cytoplasm of cells. See
http://scifun.chem.wisc.edu/chemweek/biobuff/biobuffers.html for more information on the buffer
system!
1. Write equations for the dissolving KH2PO4 and K2HPO4.
2. When mixed, the H2PO4- and the HPO42- ions will come to equilibrium. At that point, the pH of
the solution will stabilize. Write the equilibrium chemical equation as the decomposition of the
H2PO4- ion, Ka=6.23E-8.
3. After 100mL each of 0.200M KH2PO4(aq) and K2HPO4(aq) are mixed and allowed to come to
equilibrium in a closed system, it is found that the concentration of the H+ hydrogen ion is 6.23 x
10-8 M. Use an ICE table to determine the equilibrium concentrations of the other ions.
4. Calculate the pH of the buffer.
5. Write the equilibrium law equation and substitute the equilibrium constant for the reaction.
What does the equilibrium constant tell you about the extent of the forward reaction?
6. How are the equilibrium law equation and equilibrium constant for the reverse reaction
different?
7. Apply Le Chatelier’s principle to the balanced chemical equation of the system at equilibrium.
Describe the response of the buffer system to the addition of another weak acid solution so that
the hydrogen ion concentration is increased to 6.50 x 10-6M. What do you expect to occur?
8. The reaction quotient Q (see p.449) is calculated in the same way as the equilibrium constant,
but the system does not need to be at equilibrium. In fact, comparing the Q value to the
equilibrium constant enables you to determine if the system is at equilibrium. See figure 4
p.450.
 How does the Q statistic work? What information does it convey?
 Use the reaction quotient Q to determine if the system is at equilibrium immediately after
the weak acid has been added (so that [H+] = 6.50 x 10-6M) but before any reaction begins. If
it is not, in which direction will equilibrium shift?