Question

Part A: Select any systems that could be used to buffer a solution at a pH of 6. Mark all appropriate answers.

Part B: Select any systems that could be used to buffer a solution at a pH of 8.5.  Mark all appropriate answers.

		HF/F- Ka= 7.2 x 10-4
		HCO3-/ CO32- Ka=4.8 x 10-11
		HSO4-/SO42-         Ka=1.02 x 10-7
		C6H5COOH/C6H5COO- Ka=6.46 x 10-5
		H2CO3/HCO3- Ka=4.3 x 10-7
		NH4+/NH3 Ka=5.6 x 10-10

Answer 1

A)

for best buffer pH should be closest to pKa

here, pH = 6

so,

pKa should be closest to 6

use:

pKa = -log Ka

6.0 = -log Ka

Ka = 1*10^-6

So, Ka should be closest possible to 1*10^-6

This is the case for C6H5COOH/C6H5COO-

Answer: C6H5COOH/C6H5COO-

B)

for best buffer pH should be closest to pKa

here, pH = 8.5

so,

pKa should be closest to 8.5

use:

pKa = -log Ka

8.5 = -log Ka

Ka = 3.162*10^-9

So, Ka should be closest possible to 3.162*10^-9

This is the case for NH4+/NH3

Answer: NH4+/NH3