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.

you to solve for the unknown quantity.

[salt]

pH = pKa + log --------

[acid]

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.

0.020 mol/L

pH = 4.76 + log ------------

0.010 mol/L

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.

you to solve for the unknown quantity. Note that you are dealing with a basic buffer

from which you must determine pH.

[salt]

pOH = pKb + log ------

[base]

MD0837

9-16