First, d etermine the correct A - /HA ratio: The pK a of acetic acid is 4.76, an
ID: 1063469 • Letter: F
Question
First, determine the correct A-/HA ratio: The pKa of acetic acid is 4.76, and the buffer solution needs to maintain a pH of 4.25. The concentration of acetate (CH3CO2-) should be X times that of the concentration of acetic acid (CH3CO2H). What is X?
Once the correct ratio of A- to HA is known, determine the actual concentration of A- needed to make a buffer at a particular concentration.
The ratio of acetate to acetic acid in a buffer is 0.176. That is, the concentration of acetate should be 0.176 times that of the concentration of acetic acid. What is the concentration of acetate (in M) needed to prepare a 0.416 M buffer using acetic acid and acetate?
Explanation / Answer
pH = pKa + log {[CH3COO-]/[CH3COOH]}
4.25 = 4.76 + log ([CH3COO-]/[CH3COOH])
[CH3COO-]/[CH3COOH] = 10^(-0.51) = 0.309
X is 0.309
[CH3COO-]/[CH3COOH] = 0.309
[CH3COO-] = 0.309*[CH3COOH]
given,
[CH3COO-] + [CH3COOH] = 0.416 M
0.309*[CH3COOH] +[ CH3COOH] = 0.416
[ CH3COOH] = 0.318 M
[CH3COO-] = 0.309*[CH3COOH]
= 0.309*0.318 M
= 0.098 M
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