Problem: Which weak acid would be best to use when preparing a buffer solution with a pH of 8.20? a) An acid with Ka = 1.0 x10-3 M b) An acid with Ka = 1.0x1O-6 M c) An acid with Ka = 1.3x10-5 M d) An acid with Ka = 6.6 x10-9 M e) An acid with Ka = 4.0 X 10-10 M f) An acid with Ka = 7.9 X 10-5 M

FREE Expert Solution

maximum buffering capacity:              pH = pK_a

buffering range:              pH = pKa ± 1

$\boxed{{\mathbf{pK}}_{\mathbf{a}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{-}\mathbf{log}\mathbf{ }{\mathbf{K}}_{\mathbf{a}}}$

a) 

${\mathbf{pK}}_{\mathbf{a}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{-}\mathbf{log}(1.0\times {10}^{-3})$ = 3.00

buffering range: 2.oo to 4.00

b) 

${\mathbf{pK}}_{\mathbf{a}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{-}\mathbf{log}\mathbf{(}\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{6}}\mathbf{)}$ = 6.00

buffering range: 5.00 to 7.00

c) 

${\mathbf{pK}}_{\mathbf{a}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{-}\mathbf{log}\mathbf{(}\mathbf{1}\mathbf{.}\mathbf{3}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{5}}\mathbf{)}$ = 4.87

buffering range: 3.87 to 5.87