Question

determine the percentage of pentachlorophenol and pentachlorophenate at ph 2.75 and ph 3.75

Answer 1

pentachlorophenol has pka = 4.7

pH = pka + log [ conjugate base] / [ acid]   where conjugate base is pentachlorphenate

when pH = 2.75 we get

2.75 = 4.7 + log [ pentachlorophenate] / [ pentachhlorophenol]

[pentachlorophenate] = 0.01122 [ pentachlorphenol]

Thus if [pentachlorpehnl] = 1M we have [pentahcloropehante] = 0.01122M

total buffer concentration = 1M+ 0.01122 = 1.01122M

Percentage of pentachlorophenate= ( 100 x 0.001122 / ( 1.01122) ) = 1%

pentachlorphenil % = 100 - 1. = 99 % ( rounded value)

when pH = 3.75 we get

3.75 = 4.7 + log [penatchlorphenate] / [ pentachlorphenol]

[penatchlorphenate] = 0.1122 [pentachlorophenol]

% of pentachlorophenate = ( 100 x 0.1122 /1.1122) = 10 %

% of pentachlorphenol = 90 %