Question

Design a buffer that has a pH of 6.66 using one of the weak acid/conjugate base systems shown below.

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

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

Answer 1

Using the Henderson-Hasselbalch equation:

Now, a rule of thumb in selecting a desirable system is to select a system with pKa closest to the desired pH.

Thus we select the system.

To make 1L 1M [A-] salt needed = 1 mole of K2HPO4 = 174.2 g

To make 1L 3.55 M [HA] salt needed = 3.55 mole of KH2PO4 = 3.55 x 136.086 = 483.1 g