Question

You are asked to prepare 500. mL of a 0.300 M acetate buffer at pH 5.00 using only pure acetic acid (MW=60.05 g/mol, pKa=4.76), 3.00 M NaOH, and water. Answer the following questions regarding the preparation of the buffer.

1. How many grams of acetic acid will you need to prepare the 500 mL buffer? Note that the given concentration of acetate refers to the concentration of all acetate species in solution.

2. What volume of 3.00 M NaOH must you add to the acetic acid to achieve a buffer with a pH of 5.00 at a final volume of 500 mL? (Ignore activity coefficients.)

Answer 1

First you need to calculate the radius between the A^- and HA

pH = pKa + logA^-/HA

5 - 4.76 = logA^-/A^-

10^0.24 = A/HA

A^-/HA = 1.74

The total moles of HA and A are: 0.300 x 0.5 = 0.15 moles

mol A + mol HA = 0.15

A/HA = 1.74 ------> A = 1.74HA

1.74HA + HA = 0.15

2.74HA = 0.15

HA = 0.055

A = 0.15 - 0.055 = 0.095 moles

Now, with the moles of buffer needed (0.15 moles) the mass required is:

m = 0.15 x 60.05 = 9.0075 g

to the final volume of NaOH:

moles of A = 0.095 moles

V = moles /M = 0.095 / 3 = 0.03166 L or 31.66 mL