Consider how best to prepare one liter of a buffer solution with pH = 7.78 using

Question

Consider how best to prepare one liter of a buffer solution with pH = 7.78 using one of the weak acid/conjugate base systems shown here.

HC2O4-

C2O42-

6.4 x 10-5

4.19

H2PO4-

HPO42-

6.2 x 10-8

7.21

HCO3-

CO32-

4.8 x 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 =

grams potassium salt of conjugate base =

Weak Acid Conjugate Base Ka pKa

HC2O4-

C2O42-

6.4 x 10-5

4.19

H2PO4-

HPO42-

6.2 x 10-8

7.21

HCO3-

CO32-

4.8 x 10-11

10.32

Explanation / Answer

pH = 7.78

pKa = 7.21

pH = pKa + log [K2HPO4 / KH2PO4]

7.78 = 7.21 + log [K2HPO4 / KH2PO4]

[K2HPO4 / KH2PO4] = 3.715

1 / KH2PO4 = 3.715

KH2PO4 = 0.269 moles

mass of KH2PO4 = 36.6 g = weak acid

mass of K2HPO4 = 174 g = conjugate base

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