Question

You are asked to prepare 500. mL of a 0.150 M acetate buffer at pH 4.90 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 4.90 at a final volume of 500 mL? (Ignore activity coefficients.)

3. Pour the beaker contents into a 500 mL volumetric flask. Rinse the beaker several times and add the rinses to the flask. Swirl the flask to mix the solution. Add water to the mark to make the volume 500 mL. Stopper the flask and invert how many times to ensure complete mixing?

Answer 1

To prepare 500 ml of 1.5 M acetate buffer (pH = 4.90, pKa 4.76)

Using Hendersen-Hasselbalck equation,

pH = pKa + log(acetate/acetic acid)

4.90 = 4.76 + log(acetate/acetic acid)

(acetate) = 1.4(acetic acid)

we have,

(acetic acid) + (acetate) = 0.150 M x 0.5 L

or,

(acetic acid) + 1.4(acetic acid) = 1.5 M x 0.5 L = 0.75 mol

(acetic acid) = 0.75 mol/2.4 = 0.3125 mol

(acetate) = 0.75 - 0.3125 = 0.4375 mol

1. grams of acetic acid needed = 0.3125 mol x 60.05 g/mol = 18.8 g

2. Volume of 3.0 M NaOH needed = 0.4375 mol/3.0 M = 0.146 L = 146 ml

3. Mix 18.8 g of aceti acid with 146 ml of 3.0 M NaOH solution and dilute the solution to 500 ml mark to prepare this buffer.