Question

You have access to phosphoric acid (pKa1,2 = 2.1, 7.2), formic acid, acetic acid, and their conjugate bases. You are asked to make 2 different buffers with pH 3.5 and 7.4. What acids and conjugate bases do you choose? If both buffers have 0.1 M total concentration ([acid] + [conjugate base]), calculate acid and conjugate base concentrations needed to make the buffers.

Answer 1

pKa values of phosphoric acid = 2.1 , 7.2 , 12.1

pKa values of acetic acid = 4.74

pKa values of formic acid = 3.74

solution :

for pH = 3.5 buffer formic acid (pKa = 3.74) is needed.

acid = HCOOH

conjugate base = HCOO-

pH = pKa + log [conjugate base / acid]

3.5 = 3.74 + log [HCOO- / HCOOH]

[HCOO- / HCOOH] = 0.575

total concentration = 0.1 M

[HCOO- + HCOOH] = 0.1 M

HCOO- = 0.575 HCOOH

[HCOOH] = 0.0635 M

[HCOO-] = 0.0365 M

for pH = 7.4 buffer

acid = H2PO4-

conjugate base = HPO42-

pH = pKa + log [conjugate base / acid]

7.4 = 7.2 + log [ HPO42- / H2PO4-]

[ HPO42- / H2PO4-] = 1.585

total concentration = 0.1 M

[ HPO42- + H2PO4-] = 0.1 M

HPO42- = 1.585 H2PO4-

[H2PO4-] = 0.0387 M

[HPO42-] = 0.0613 M