Question

Design a buffer that has a pH of 3.73 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 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 sodium salt of weak acid = grams sodium salt of conjugate base =

Answer #1

Buffer preparation

pH needed of buffer = 3.73

pKa of weak acid chosen to prepare the buffer should be closest to the pH of the buffer

So the combination we would use = HC2O4-/C2O4^2- (weak acid/conjugate base system) with pKa = 4.19

Using Hendersen Hasselbalck equation,

pH = pKa + log(base/acid)

3.73 = 4.19 + log(C2O4^2-/HC2O4-)

(C2O4^2-) = 0.35(HC2O4-)

We know concentration of buffer solution,

(HC2O4-) + (C2O4^2-) = 1.00 M x 1.00 L = 1 mol

substituting,

(HC2O4-) + 0.35(HC2O4-) = 1 mol

(HC2O4-) = 1/1.35 = 0.741 mol

So,

(C2O4^2-) = 1 - 0.741 = 0.259 mol

and,

grams of weak acid (HC2O4-) needed = 0.741 mol x 112.0167 g/mol = 83.004 g

grams of conjugate base (C2O4^2-) needed = 0.259 mol x 134 g/mol = 34.706 g

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