SOLVING BUFFER PROBLEMS
The most important consideration when preparing a buffer is determining
whether an acidic or basic buffer is being prepared. Examine the components of the
buffer. If a weak acid and its salt are the components, use the expression for acidic
buffers. If a weak base and its salt are used, select the expression for basic buffers.
a. Example 1. Calculate the pH of a buffer solution that contains 0.010 mol/L
acetic acid and 0.020 mol/L sodium acetate. The Ka for acetic acid is 1.75 X 10-5.
Solution. Read the problem carefully, and select the expression that allows
you to solve for the unknown quantity.
pH = pKa + log --------
Make any necessary conversions.
pKa = -log Ka
pKa = -log (1.75 X 10-5)
pKa = 4.76
Substitute the given information, and solve for the unknown quantity.
pH = 4.76 + log ------------
pH = 4.76 + 0.301
pH = 5.06
b. Example 2. What is the pH of a buffer solution that contains 1.50 X 10-3
mol/L NH4Cl and 2.00 X 10-4 mol/L NH4OH? The pKb for ammonium hydroxide is 4.75.
Solution Read the problem carefully, and select the expression that allows
you to solve for the unknown quantity. Note that you are dealing with a basic buffer
from which you must determine pH.
pOH = pKb + log ------