Design a buffer that has a pH of 8.08 using one of the weak acid/conjugate base

Question

Design a buffer that has a pH of 8.08 using one of the weak acid/conjugate base systems shown below.

HC2O4-

C2O42-

6.4×10-5

4.19

H2PO4-

HPO42-

6.2×10-8

7.21

HCO3-

CO32-

4.8×10-11

10.32

How many grams of the potassium salt of the weak acid must be combined with how many grams of the potassium salt of its conjugate base, to produce 1.00 L of a buffer that is 1.00 M in the weak base?

grams potassium salt of weak acid =  g

grams potassium salt of conjugate base =  g

Weak Acid Conjugate Base Ka pKa

HC2O4-

C2O42-

6.4×10-5

4.19

H2PO4-

HPO42-

6.2×10-8

7.21

HCO3-

CO32-

4.8×10-11

10.32

Explanation / Answer

for pH = 8.08, choose a pKa value for the buffer near that value

best option is H2PO4- and HPO4-2 wit pKa = 7.21

so

pH = pKa + log(HPO4-2/HH2PO4-)

find mass fo V = 1 L and total M = 1

so

mol = 1

mol of HPO4-2 + mol of H2PO4- = 1

from the pH ratio

8.08 = 7.21 + log(HPO3-2/H2PO4-)

7.4131 = HPO4-2/H2PO4-

HPO4-2 = 7.4131 * H2PO4-

HPO3-2 + H2PO4- = 1

7.4131 * H2PO4- + H2PO4- = 1

H2PO4- = 1/(8.4131) = 0.118862250

so

HPO4-2 = 1-0.118862250 = 0.88113775

so

MW of KH2PO4

mol of KH2PO4 = 0.118862250*136.086 = 16.17548 g of KH2PO4

MW of K2HPO4

mol of K2HPO4 = 0.88113775*174.2 = 153.49 g of K2HPO4

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