If You want to make a buffer that maintains a pH of 4.55. 1) Which of the follow

Question

If You want to make a buffer that maintains a pH of 4.55.

1) Which of the following acids (Hydrochloric, Butanoic, Benzoic, Hydrocyanic) would you use for the buffer for the best choice? Why is it best choice?

2) Name a second compound that you would use to make your buffer.

c) What should the ratio of the concentrations be for the two compounds you will use to make your buffer?

d) You want the total concentration of both compounds to be 1.00 M. What should the concentration of each compound be?

Explanation / Answer

1) The pKa of HCl = -7

The pKa of butanoic acid = 4.82

The pKa of benzoic acid = 4.20

The pKa of HCN = 9.31

Here, the pKa value of butanoic acid is more close to 4.55. (The difference = 4.82 - 4.55 = 0.27)

Therefore, butanoic acid is the best choice for making a buffer that maintains a pH of 4.55.

2) The second compound that you would use to make your buffer is the corresponding conjugate base, i.e. sodium butanoate.

c) According to Henderson-Hasselbulch equation: pH = pKa + Log([sodium butanoate]/[butanoic acid])

i.e. 4.55 = 4.82 + Log([sodium butanoate]/[butanoic acid])

i.e.  Log([butanoic acid]/[sodium butanoate]) = 0.27

i.e. [butanoic acid]/[sodium butanoate] = 100.27 = 1.862

i.e. [butanoic acid] = 1.862*[sodium butanoate]

d) According to the given condition: [butanoic acid] + [sodium butanoate] = 0.1

i.e. 1.862*[sodium butanoate] +  [sodium butanoate] = 0.1

i.e. [sodium butanoate] = 0.1/2.862 = 0.0349 M

Therefore, [butanoic acid] = 0.1 - 0.0349 = 0.0651 M

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