Question

Which of the following has the best buffering capacity. (Buffering capacity can be thought of as the amount of strong acid or base that must be added to a buffered solution to change its pH by 1)? Select one: a. 0.4 M CH3COONa/ 0.2 M CH3COOH b. None of these are appropriate concentrations for buffers c. they all have the same buffering capacity d. 0.4 M CH3COONa/ 0.6 M CH3COOH Correct e. 0.3 M CH3COONa/ 0.6 M CH3COOH

Answer 1

Answer:

The buffer d has the best buffering capacity.

Explanation:

It is possible to obtain the pH of a buffer using Henderson-Hasselbalch formula:

pH = pka + log₁₀ [A⁻]/[HA]

For CH₃COOH/CH₃COONa buffer:

pH = 4,8 + log₁₀ [CH₃COONa]/[CH₃COOH]

a. pH of this buffer is:

pH = 4,8 + log₁₀ [0,4]/[0,2]

pH = 5,1

As buffering capacity can be thought of as the amount of strong acid that must be added to a buffered solution to change its pH by 1:

For a pH of 4,1:

4,1 = 4,8 + log₁₀ [0,4-x]/[0,2+x]

Where x are the moles of strong acid added.

0,200 = [0,4-x]/[0,2+x]

0,0400 + 0,2x = 0,4 - x

x = 0,3 mol

d. pH of this buffer is:

pH = 4,8 + log₁₀ [0,4]/[0,6]

pH = 4,62

For a pH of 3,62:

3,62 = 4,8 + log₁₀ [0,4-x]/[0,6+x]

Where x are the moles of strong acid added.

0,066 = [0,4-x]/[0,6+x]

0,0396 + 0,066x = 0,4 - x

x = 0,338 mol

e. pH of this buffer is:

pH = 4,8 + log₁₀ [0,3]/[0,6]

pH = 4,5

For a pH of 3,5:

3,5 = 4,8 + log₁₀ [0,3-x]/[0,6+x]

Where x are the moles of strong acid added.

0,050 = [0,3-x]/[0,6+x]

0,030 + 0,05x = 0,3 - x

x = 0,257 mol

Thus, buffer d needs more strong acid to change its pH. That means that have the best buffering capacity

You can do the same process using strong base (Increasing pH in 1) and you will obtain the same results!

I hope it helps!