[yankeekd25]

An aqueous solution contains 0.174 M carbonic acid and 0.165 M perchloric acid.

Calculate the carbonate ion concentration in this solution.

Completely stumped.

[Borek]

K=[H⁺]²[CO₃^2-]/[H₂CO₃]

[yankeekd25]

Do you use the H+ from the polyprotic or from the strong acid? Do you use the first Ka or the second Ka of the polyprotic acid?

[AWK]

At the beginning you know only [H⁺] coming from a strong acid (and usally this concentration is sufficient for calculations of carbonate anion.
K_a1K_a2=[H⁺]²[CO₃^2-]/[H₂CO₃]

[yankeekd25]

At the beginning you know only [H⁺] coming from a strong acid (and usally this concentration is sufficient for calculations of carbonate anion.
K_a1K_a2=[H⁺]²[CO₃^2-]/[H₂CO₃]

An aqueous solution contains 0.174 M carbonic acid and 0.165 M perchloric acid.

Calculate the carbonate ion concentration in this solution.

Completely stumped.
 
[H+] for the strong acid = (Ka x initial concentration)^1/2 correct? I looked online for the Ka of HClO4 and all I get is "large."

Then I find the [H2CO3] using an ice table?

So:
H2CO3---> H+ + HCO3-
HCO3- --> H+ + CO3 2-

Ka for carbonic- 4.2E-7 and 4.8E-11
4.2E-7= x^2/ .174  ??

Am I okay so far?

[Borek]

You may safely assume HClO₄ is fully dissociated, that's what "large" means (http://www.chembuddy.com/?left=BATE&right=dissociation_constants - it is about 10⁷).

You my also safely assume H₂CO₃ is not dissociated at all - it is a weak acid, in the low pH solution, fully protonated.

Now check the overall dissociation constant - how many unknowns in the equation?

[yankeekd25]

You may safely assume HClO₄ is fully dissociated, that's what "large" means (http://www.chembuddy.com/?left=BATE&right=dissociation_constants - it is about 10⁷).

You my also safely assume H₂CO₃ is not dissociated at all - it is a weak acid, in the low pH solution, fully protonated.

Now check the overall dissociation constant - how many unknowns in the equation?

So do I actually have to do anything the strong acid in terms of the calculations?

[Borek]

I assume you mean "with the strong acid". No. It is just there as a source of H⁺ in a given concentration.

[yankeekd25]

K=[H⁺]²[CO₃^2-]/[H₂CO₃]

An aqueous solution contains 0.174 M carbonic acid and 0.165 M perchloric acid.

Calculate the carbonate ion concentration in this solution.

H2CO3 --> H+ HCO3-
.174          x      x

x^2/ .174= 4.2E-7 x= 2.48E-4


4.2E-7 (4.8E-11)= [2.48E-4]^2 [CO3 2-] / .174

3.50784E-18 = 6.1504E-8 [CO3 2-]
[CO3 2-] = 5.70E-11

This is for OWL online, and I typed this in as the answer, and it was incorrect.

[Borek]

I have no idea what these numbers mean, but 5.7e-11 is incorrect.

All you have to do is to plug numbers into the overall dissociation constant equation. No ICE tables, nothing.

[yankeekd25]

I didn't realize that I'm supposed to use the [strong acid] as the concentration of the H+. (DUH). Thanks for all your help Borek. I really do appreciate it.