Question

You are asked to prepare 500. mL of a 0.100 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.)

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

Concepts and reason

An aqueous solution consists of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid which is represented as a buffer solution. These buffer solutions can maintain pH at a nearly constant value, upon addition of small amount of strong acid or base.

Fundamentals

The pH can be calculated by the Henderson – Hasselbalch equation.

The concentration can be calculated by,

(1)

Mass of acetic acid was calculated as follows

[Part 1]

Part 1

(2)

Number of moles of sodium acetate required to attain the buffer with $pH=5.00 $ and volume of was calculated as

(3)

Swirl the flask till it dissolves completely.

Ans: Part 1