Chemistry 141
G. E. Hofmeister
Calculating equilibrium constants from pKa values
OH
?
+ CH3CH2O Na
1.
Identify the acid and base in the reactants and write the conjugate base/acid products:
OH
O Na
+ CH3CH2OH
+ CH3CH2O Na
acid
2.
base
conj. base
Look up the pKa values of both acids and write the equilibrium reaction that these values
represent: pKa(C6 H5 OH) = 10; pKa(CH 3 CH2 OH) = 15.9
OH
O
+ H2O
CH3CH2OH + H2O
3.
conj. acid
+ H3O
CH3CH2O
+ H3O
Ka =
[C6H5O ][H3O ]
= 10-10
[C6H5OH]
Ka = [CH3CH2O ][H3O ] = 10-15.9
[CH3CH2OH]
Figure out how the original acid/base equation can be expressed as the addition of the two
Ka equations :
OH
O
+ H2O
CH3CH2O
+ H3O
Net: OH
+ H3O
CH3CH2OH + H2O
K = [C6H5O ][H3O ] = 10-10
[C6H5OH]
K=
[CH3CH2OH]
=
[CH3CH2O ][H3O ]
K=
-10
[C6H5O ] [CH3CH2OH]
= 10-15.9
[C6H5OH] [CH3CH2O ]
10
1
10-15.9
O
+ CH3CH2O
+ CH3CH2OH
Therefore, the equilibrium constant for this reaction = 105.9