Question

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

Answer 1

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.)

Answer 2

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!

Answer 3

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.

Answer 4

Yes both HCN or HOCl were correct. Thank you so much!