Question

A student was tasked to prepare a 500ml acetic acid buffer of concentration 10M with a pH of 4.5. (Molar mass of acetic acid = 60.05g/mol, sodium acetate = 83.03g/mol, pKa of acetic acid = 4.76 at 25 degrees Celsius, NaOH = 40g/mol).

What is the next step the student should take to prepare the buffer?

a) Add acetic acid only

b) Add sodium acetate only

c) Add a mixture of acetic acid and sodium acetate

d) Add NaOH

Answer 1

The student should add a mixture of acetic acid and sodium acetate to prepare a 10M acetic acid buffer with a pH of 4.5. The Henderson-Hasselbalch equation is used for calculating the precise amounts needed.

Step-by-step explanation:

The next step the student should take to prepare a 500ml acetic acid buffer of concentration 10M with a pH of 4.5 is to add a mixture of acetic acid and sodium acetate. This is because a buffer solution consists of a mixture of a weak acid and its conjugate base. To achieve the desired pH, which is below the pKa value of acetic acid (4.76), the ratio of acetic acid to sodium acetate must be greater than 1. However, since the problem does not specify the amount of each component in the mix, further calculations using the Henderson-Hasselbalch equation would be needed to determine the precise amounts of acetic acid (CH3COOH) and sodium acetate (CH3COONa) to add. The Henderson-Hasselbalch equation is pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base (sodium acetate) and [HA] is the concentration of the acid (acetic acid).