pKa question?

Question

The ph of 0.1 M HClO2(chlorous acid) aqueous solution was measured to be 1.2. What is the value of pKa for chlorous acid?

1.2.57

2. .11

3. 3.91

4. .96

5. 1.40

6. 1.20

Answer 1

HClO2 --> H+ + ClO2-

pH = 1.2, 10^-1.2 = H+ = 0.063

ClO2- = H+ = 0.063

HClO2 = 0.10 - 0.063 = 0.037

Ka = (0.063)2/0.037 = 1.1 X 10^-1 = 0.11

pKa = -log(0.11) = 0.96

Answer 2

pH is 1.2

[H+] = 10^-1.2 (definition) = 0.063

So of the 0.1 M HClO2 you started with, you have 0.037 still there, and 0.063 have dissociated.

Ka = [H+][ClO2 -]/[HClO2]

Every time you dissociate HClO2, you make ClO2 - as well as H+

So Ka = [0.063][0.063]/[0.037]

and pKa = - log Ka

Answer 3

the answer is .96 in my calculator(casio fx991MS), the formula is:
1.)shift+log-1.2=
2.)you'll get the Ans 0.063095734.......then press
Ans^2/(0.1-Ans)= 0.11
3.) -log0.11=0.96