Given the following table of pKa values, at a pH of 4.5 which substance exists predominantly in the protonated form?
Acid pKa Value
HNO3 -1.3
HSCN 4.0
HCN 9.4
HoCl 7.5
A. Only HNO3
B. Both HCN and HOCl
C. Both HNO3 and HSCN
D. Only HCN
Is the answer for this one B or D. I'm not entirely sure
Yes both HCN or HOCl were correct. Thank you so much!
If you used the HH equation, then base/acid ratio is what you want. For a generic acid, say HA, it ionized as HA ==>H^+ A^- so the HA is the acid and that's the protonated form while A^- is the base and that's the unprotonated form. So if your ratio of b/a for HCN = 1.3E-5 or base = 1.3E-5*a. For HCN, then, the base(unprotonataed) = 1.3E-5*acid(protonated) so for every 130,000 protonated you have unprotonated. That looks to me like predominately in the form of protonated.
For HOCl that ratio is b/a = 0.001 or for every 1000 protonated you have 1 unprotonated. I would call 1 to 1000 predominate so I would choose both. But as I said above, it depends upon what the author of the question calls predominate. I would go with both.
You're right, The choice comes down to B or D. You must have calculated the ratio of the protonated form to the unprotonated form. What values did you get for HOCl and HCN? (Part of the problem, I think, is that the term "predominately" is a relative term and we can't be sure what the author of the problem considers predominate.)
For the ratio of Base/Acid for HCN, I got 1.26E-5.
For the ratio of Base/Acid for HOCl, I got 0.001.
I used the Henderson-Hasslebalch equation for this. I don't know how to decide between whether it is both or just one of them. Also what exactly does this ratio tell about what form it would be in. Thanks!
To determine which substance exists predominantly in the protonated form at a pH of 4.5, we need to compare the pKa values to the pH.
First, let's understand what pKa represents. The pKa value is a measure of the strength of an acid. It is the pH at which half of the acid has dissociated into its conjugate base and the remaining half is still in its protonated form.
In this case, we are given a pH of 4.5.
To decide which compounds exist predominantly in the protonated form, we need to find the compounds whose pKa values are lower than the pH of 4.5.
Comparing the pKa values to a pH of 4.5, we see that:
- HNO3 has a pKa value of -1.3, which is below 4.5
- HSCN has a pKa value of 4.0, which is also below 4.5
- HCN has a pKa value of 9.4, which is above 4.5
- HoCl has a pKa value of 7.5, which is above 4.5
From this comparison, we can conclude that only substances HNO3 and HSCN exist predominantly in the protonated form at a pH of 4.5.
Therefore, the correct answer is C. Both HNO3 and HSCN.