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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!

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For any mono-protic acid, when the acid dissociates in water, every H+ created will create one CB-.

H-Cit <--> 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+].

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  • $\begingroup$ Nice. Down vote with no comment? $\endgroup$ – MarsJarsGuitars-n-Chars Apr 22 '18 at 13:13
  • $\begingroup$ I didn't downvote or upvote but your answer is not complete it's more of a guide of now. $\endgroup$ – Avnish Kabaj Apr 22 '18 at 13:44
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    $\begingroup$ @AvnishKabaj Thanks. For homework type questions, I believe we don't want to just answer, but give information about the general principle at work? $\endgroup$ – MarsJarsGuitars-n-Chars Apr 22 '18 at 13:46
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    $\begingroup$ Yes that makes a complete answer. $\endgroup$ – Avnish Kabaj Apr 22 '18 at 13:57

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