In: Chemistry
Calculate the pH of 0.10 M sodium acetate solution ( Ka, acetic acid = 1.76 x 10-5 ).
use:
Kb = Kw/Ka
Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC
Kb = (1.0*10^-14)/Ka
Kb = (1.0*10^-14)/1.8*10^-5
Kb = 5.556*10^-10
CH3COO- dissociates as
CH3COO- + H2O -----> CH3COOH + OH-
0.1 0 0
0.1-x x x
Kb = [CH3COOH][OH-]/[CH3COO-]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((5.556*10^-10)*0.1) = 7.454*10^-6
since c is much greater than x, our assumption is correct
so, x = 7.454*10^-6 M
use:
pOH = -log [OH-]
= -log (7.454*10^-6)
= 5.1276
use:
PH = 14 - pOH
= 14 - 5.1276
= 8.8724
Answer: 8.87