Calculate the concentration of H3O
+ present
in 0.377 M H2CO3.
Answer in units of mol/L
I know the Ka of H2CO3 is 4.3E-7. But I don't know what to do from there. (Or, I'm not getting the correct answer at least)
When you have problems like this post your work. That way we can see at a glance what is wrong.
Well, I've tried going using 4.3e-7=x^2/0.377 to get to what x equals, I've been getting 0.0069, but it's incorrect.
Once again I was foolish and mistyped an earlier answer, getting 4.02e-4 for my answer. I had not put in the 0 between the 4 and the 2 when submitting.
Your set up is ok. My answer is 4.026E-4M. However, you're allowed only two significant figures (set by the 4.3E-7 Ka value) so I would round that to 4.0E-4M. I suspect the s.f. thing is catching you.
To calculate the concentration of H3O+ in a solution of H2CO3, you need to use the given Ka value and the stoichiometry of the acid dissociation reaction. This reaction can be represented as:
H2CO3 + H2O ⇌ H3O+ + HCO3-
Since H2CO3 is a weak acid, it partially ionizes in water, yielding H3O+ and HCO3-. The Ka value is the acid dissociation constant and represents the equilibrium constant for this reaction. The equation for Ka is:
Ka = [H3O+] [HCO3-] / [H2CO3]
We can assume that the concentration of HCO3- will be negligible compared to that of H2CO3 because H2CO3 is a much stronger acid than HCO3-. Therefore, we can simplify the equation to:
Ka ≈ [H3O+] [HCO3-] / [H2CO3] ≈ [H3O+] [HCO3-] / (0.377 M)
Now, we need to rearrange the equation to solve for [H3O+]. Since we know the value of Ka and the concentration of H2CO3, we can substitute them and solve for [H3O+]:
[H3O+] = (Ka * [H2CO3]) / [HCO3-]
[H2CO3] is given as 0.377 M, and [HCO3-] can be considered negligible. Therefore, we can disregard [HCO3-] and calculate [H3O+] as:
[H3O+] = (4.3E-7 * 0.377 M) / (negligible)
Since [HCO3-] is negligible, it can be considered as 0. Therefore:
[H3O+] ≈ (4.3E-7 * 0.377 M) / 0 = 0
Hence, the concentration of H3O+ in a 0.377 M H2CO3 solution is approximately 0 mol/L.
It is important to note that the concentration of H3O+ in this case is negligible because H2CO3 is a weak acid and does not fully dissociate in water.