The Ka of a monoprotic weak acid is 5.82 × 10-3. What is the percent ionization of a 0.107 M solution of this acid?
Lets write the dissociation equation of HA
HA -----> H+ + A-
0.107 0 0
0.107-x x x
Ka = [H+][A-]/[HA]
Ka = x*x/(c-x)
Assuming small x approximation, that is lets assume that x can be ignored as compared to c
So, above expression becomes
Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((5.82*10^-3)*0.107) = 2.495*10^-2
since x is comparable c, our assumption is not correct
we need to solve this using Quadratic equation
Ka = x*x/(c-x)
5.82*10^-3 = x^2/(0.107-x)
6.227*10^-4 - 5.82*10^-3 *x = x^2
x^2 + 5.82*10^-3 *x-6.227*10^-4 = 0
Let's solve this quadratic equation
Comparing it with general form: (ax^2+bx+c=0)
a = 1
b = 5.82*10^-3
c = -6.227*10^-4
solution of quadratic equation is found by below formula
x = {-b + √(b^2-4*a*c)}/2a
x = {-b - √(b^2-4*a*c)}/2a
b^2-4*a*c = 2.525*10^-3
putting value of d, solution can be written as:
x = {-5.82*10^-3 + √(2.525*10^-3)}/2
x = {-5.82*10^-3 - √(2.525*10^-3)}/2
solutions are :
x = 2.221*10^-2 and x = -2.803*10^-2
since x can't be negative, the possible value of x is
x = 2.221*10^-2
% dissociation = (x*100)/c
= 2.221*10^-2*100/0.107
= 20.7606 %
Answer: 20.8 %
Feel free to comment below if you have any doubts or if this answer do not work. I will edit it if you let me know
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