Example on how to make up a buffer: A student has 100 mL of 0.1 M acetic acid an
ID: 609413 • Letter: E
Question
Example on how to make up a buffer: A student has 100 mL of 0.1 M acetic acid and has plenty of 0.1 M sodium acetate and wants to make up a buffer with pH = 5.1. Using the pKa of acetic acid/sodium acetate 4.74 and the equation pH = pKa + log[base]/[acid] the student finds the antilog of the ratio of acid and base and figures out for every 1 mL of acid she needs 2.29 mL of acid.
What is the final concentration (in molarity) of the sodium acetate and the acetic acid. Prove that the student has correctly calculated the volume of sodium acetate that she needs.
Explanation / Answer
First calculate the molar concentration of the two compounds in the final buffer solution:
Final volume = 35+10 = 45mL
1) CH3COONa
M1V1 = M2V2
M1*45 = 0.1*35
M1 = 3.5/45
M1 = 0.078M
2) CH3COOH
M1V1 = M2V2
M1*45 = 0.2*10
M1 = 2/45
M1 = 0.044M
In order to calculate pH use the Henderson Hasselbalch equation:
pKa = -log Ka = -log 1.8*10^-5 = 4.74
pH = pKa + log([salt]/[acid]
pH = 4.74 + log ( 0.078/0.044)
pH = 4.74 + log 1.77
pH = 4.74+ 0.25
pH = 4.99
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