A certain weak acid, HA, has a Ka value of 5.5×10−7.
Part A
Calculate the percent ionization of HA in a 0.10 M solution.
Part B
Calculate the percent ionization of HA in a 0.010 M solution.
A)
HA dissociates as:
HA -----> H+ + A-
0.1 0 0
0.1-x x x
Ka = [H+][A-]/[HA]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((5.5*10^-7)*0.1) = 2.345*10^-4
since c is much greater than x, our assumption is correct
so, x = 2.345*10^-4 M
% dissociation = (x*100)/c
= 2.345*10^-4*100/0.1
= 0.2345 %
Answer: 0.234 %
B)
HA dissociates as:
HA -----> H+ + A-
1*10^-2 0 0
1*10^-2-x x x
Ka = [H+][A-]/[HA]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((5.5*10^-7)*1*10^-2) = 7.416*10^-5
since c is much greater than x, our assumption is correct
so, x = 7.416*10^-5 M
% dissociation = (x*100)/c
= 7.416*10^-5*100/0.01
= 0.7416 %
Answer: 0.742 %
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