-log Ka = pKa
For HF, pKa = - log (7.2 x 10-4) =
3.142
For HNO2, pKa = - log (4 x
10-4) = 3.397
HF is the deciding reagent for pH.
           Â
HFÂ Â Â Â Â Â
---->Â Â Â Â Â
H+Â Â Â Â + Â Â
F-
Initial       0.175
MÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0
Change    Â
-x                    Â
+x             Â
+x
Equilibrium       0.175 -
x               Â
x                Â
x
Ka = [H+][F-] / [HF]
7.2 x 10-4 = x2 / (0.175 - x)
0.00072 (0.175 - x) = x2
x = 0.0112 M
[H+] = 0.0112 M
pH = -log [H+] = 1.95
