You are asked to go into the lab and prepare a buffer solution with a pH of 6.40

Question

You are asked to go into the lab and prepare a buffer solution with a pH of 6.40 ± 0.2. Which weak acid would be the best choice?
A.carbonic acid, Ka = 4.2 x 10-7
B.potassium hydrogen phthalate, Ka = 3.1 x 10-6   
C.hydrosulfuric acid, Ka = 9.5 x 10-8    D.phenol, Ka = 1.3 x 10-10   
E.ascorbic acid, Ka = 8.0 x 10-5 You are asked to go into the lab and prepare a buffer solution with a pH of 6.40 ± 0.2. Which weak acid would be the best choice?
A.carbonic acid, Ka = 4.2 x 10-7
B.potassium hydrogen phthalate, Ka = 3.1 x 10-6   
C.hydrosulfuric acid, Ka = 9.5 x 10-8    D.phenol, Ka = 1.3 x 10-10   
E.ascorbic acid, Ka = 8.0 x 10-5
A.carbonic acid, Ka = 4.2 x 10-7
B.potassium hydrogen phthalate, Ka = 3.1 x 10-6   
C.hydrosulfuric acid, Ka = 9.5 x 10-8    D.phenol, Ka = 1.3 x 10-10   
E.ascorbic acid, Ka = 8.0 x 10-5 A.carbonic acid, Ka = 4.2 x 10-7
B.potassium hydrogen phthalate, Ka = 3.1 x 10-6   
C.hydrosulfuric acid, Ka = 9.5 x 10-8    D.phenol, Ka = 1.3 x 10-10   
E.ascorbic acid, Ka = 8.0 x 10-5 A.carbonic acid, Ka = 4.2 x 10-7
B.potassium hydrogen phthalate, Ka = 3.1 x 10-6   
C.hydrosulfuric acid, Ka = 9.5 x 10-8    D.phenol, Ka = 1.3 x 10-10   
E.ascorbic acid, Ka = 8.0 x 10-5

Explanation / Answer

we know that

for buffers with weak acid

pH = pKa + log [ salt / acid ]

now consider equal conc of salt and acid

we get

pH = pKa + log 1

pH = pKa

given

pH = 6.4

so

pKa = 6.4

we know that

pKa = -log Ka

so

-log Ka = 6.4

Ka = 4 x 10-7

so

the pKa of the acid should be within this range

from the given values

this condition satisfies for carbonic acid

so

the the best choice is

A) carbonic acid Ka = 4.2 x 10-7

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