What is the citric acid concentration, [HCit], in the lemon juice?
For any mono-protic acid, when the acid dissociates in water, every H+ created will create one CB-.


  H-Cit  &lt;--> H+ + Cit-


For each 1 H-Cit. 1 H+ and 1 Cit- is produced upon dissociation.
Therefore, the [H+] = [Cit-]

For a Ka expression, set these two [] equal to each other, and the expression for [H+]*[Cit-] = x^2

Use this to determine the amount of [Cit-] created when you create x amount of [H+].