Design a buffer that has a pH of 9.71 using one of the weak acid/conjugate base

Question

Design a buffer that has a pH of 9.71 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

pH of acidc buffer = pka + log(base/acid)

pka of HCO3^- = 10.32

9.71 = 10.32+logx

X = (base/acid) = 0.245

TOTAL NO OF MOL OF BUFFER(acid+base) = 1*1 = 1 mol

No of mol of conjugate base required = 1-0.8 = 0.2 mol

No of mol of conjugate acid required = 1/1.245 = 0.8 mol

mass of potassium salt of weak (KHCO3) required = 0.8*100 = 80 grams

mass of potassium salt of conjugate base( K2CO3) required = 0.2*138 = 27.6 grams

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