Determine the equilibrium constant, Ka of 0.12 mole HNO2 is dissolved to make 1 L which has a hydronium ion (Hydrogen ion) concentration of 0.00115 mole/liter?
I have it as Ka= [H+] [NO2-] / [HNO2]
..........HNO2 ==> H^+ + NO2^-
I..........0.12..........0...........0
C..........-x.............x...........x
E.........0.12-x.......x...........x
You have the correct Ka expression. Substitute the E line into the Ka expession you've written like this.
Ka = (H^+)(NO2^-)/(HNO2)
Ka = (x)(x)/(0.12-x)
The problem tells you x = 0.00115, Plug than in for x and solve for Ka.
Post your work if you get stuck.