What is the citric acid concentration, [HCit], in the lemon juice? The pH of lemon juice is about 2.1.  What is the citric acid concentration, [HCit], in the lemon juice?


I am given that $HCit \leftarrow \rightarrow H^+ + Cit^-$ and that $K_a = 8.4 \times 10^{-4}$. 

$K_a = \frac{[H+][Cit]}{[HCit]} = 8.4 \times 10^{-4}$, and $[H+]$ is $10^{-2.1}$. However, without knowing $[Cit]$, how can I get [HCit]? Any help would really be appreciated!
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+].