Question

A) Select any systems that could be used to buffer a solution at a pH of 5. Mark all appropriate answers.

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

B) Select any systems that could be used to buffer a solution at a pH of 9. Mark all appropriate answers.

		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
		HF/F-     Ka= 7.2 x 10-4
		NH4+/NH3 Ka=5.6 x 10-10

Answer 1

A)

A buffer works best at that pH value which is equal to pKa

Here pH = 5

So, pKa should be closer to 5 for buffer (+1 and -1 accepted)

so, Ka should be of 10^-5 order (or anywhere between 10^-6 to 10^-4)

C6H5COOH/C6H5COO- Ka=6.46 x 10-5

B)

A buffer works best at that pH value which is equal to pKa

Here pH = 9

So, pKa should be closer to 9 for buffer (+1 and -1 accepted)

so, Ka should be of 10^-9 order(or anywhere between 10^-10 to 10^-8)

NH4+/NH3 Ka=5.6 x 10-10