HZ is a weak acid. A concentration of 0.020M in aqueous solution gives a pH of 4.93 what is the Ka?

Answer 1

...............HZ ==> H^+ + Z^-
initial........0.02...0.....0
change..........-x....x......x
equil.........0.02-x..x......x

Ka = (H^+)(Z^-)/(HZ)
You know H^+ by solving pH = -log(H^+) (that's the x)
Z= x and that's the same as (H^+).
HZ = 0.02-x = 0.02-(H^+).
Substitute into Ka expression and solve for Ka. I get approximately 7E-9