In: Chemistry
determine the [OH-] and pH of a solution tht is 0.140 M in F-.
Answer –
We are given, [F-] = 0.140 M
First we need to look the Kb value for F- from the Ka value of HF
The Ka value for HF = 7.2*10-4
So, Kb for F- = 1*10-14 / Ka
= 1*10-14 / 7.2*10-4
= 1.39*10-11
Now we need to put ICE chart
F- + H2O -----> HF + OH-
I 0.140 0 0
C -x +x +x
C 0.140-x +x +x
We know,
Kb = [HF] [OH-] / [F-]
1.39*10-11 = x*x/(0.140-x)
We can neglect x in the 0.140-x, because Kb value is too small
x2 = 1.39*10-11 *0.140
= 1.92*10-12
x = 1.39*10-6 M
so, x = [OH-] = 1.39*10-6 M
now we can calculate pOH from [OH-]
pOH = -log [OH-]
= - log 1.39*10-6 M
= 5.86
So, pH = 14- pOH
= 14 -5.86
= 8.14