Question

1. Design a buffer that has a pH of 4.48 using one of the weak base/conjugate acid systems shown below.

Weak Base	Kb	Conjugate Acid	Ka	pKa
CH3NH2	4.2 x 10-4	CH3NH3+	2.4 x 10-11	10.62
C6H15O3N	5.9 x 10-7	C6H15O3NH+	1.7 x 10-8	7.77
C5H5N	1.5 x 10-9	C5H5NH+	6.7 x 10-6	5.17

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

grams chloride salt of conjugate acid =

grams weak base =

2. Design a buffer that has a pH of 3.54 using one of the weak acid/conjugate base systems shown below.

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

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

grams of sodium salt of weak acid =

grams sodium salt of conjugate base =

Answer 1

1) The base that has a pKa closest to the necessary pH is chosen, it is C5H5N.

Calculate the concentration ratio between salt and acid, using the clear equation of henderson hasselbach:

[Salt] / [Acid] = 10 ^ (pH-pKa) = 10 ^ (4.48 - 5.17) = 0.204

The acid concentration is calculated:

[Acid] = 1 M / 0.204 = 4.9 M

The grams of acid salt and base are calculated:

g Acid = M * V * MM = 4.9 M * 1 L * 115.56 g / mol = 566.24 g

g Base = 1 M * 1 L * 79.1 g / mol = 79.1 g

2) Same procedure above, choose HC2O4-, salt / acid ratio:

[Salt] / [Acid] = 10 ^ (3.54 - 4.19) = 0.224

[Salt] = 1 M * 0.224 = 0.224 M

The grams are calculated:

g Acid = 1 M * 1 L * 112.01 g / mol = 112.01 g

g Base = 0.224 M * 1 L * 111 g / mol = 24.86 g

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