Question

The pKa of one of the hydrogens that is attached to one of the nitrogens of the purine guanine is 9.7. If a solution of guanine is measured to have a pH of 10.4, then what fraction of the guanine molecules would be in the protonated form at this pH? Enter your answer as a decimal to two figures.

Answer 1

Lets initial guanine be 1 M
Guanine is represented as A- and its protonated form by HA
A- + H+   ---> HA
1                        0
1-x                     x
use:
pH = pKa + log {[A-]/[HA]}
10.4 = 9.7 + log {[A-]/[HA]}
log {[A-]/[HA]} = 0.7
[A-]/[HA] = 5.01
So,
[HA]/[A-] = 0.20
x/(1-x) = 0.20
x = 0.20 - 0.20 x
1.20 x = 0.20
x = 0.17
So, 0.17 fraction is in protonated form