Given the following table of pKa values, at a pH of 4.5 which substance exists predominantly in the protonated form?

Question

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

Someone please help. I'm not sure about the answer at all.

Answer 1

Have you worked with the Henderson-Hasslebalch equation. That's the easy way to do this and I'll use that. It can be done another way if I must.
You want to calculate the ratio of the ionized form (the base) to the unionized form (the acid and the protonated form)
The HH equation is pH = pKa + log (base)/(acid)
For HNO3 we have 4.5 = -1.3 + log (B/A)
log B/A = 4.5 + 1.3 = 5.8 and Base/acid = 6.8E5 which tells you the numerator (the ions) are huge compared with the unionized (the protonated) form. So most of the HNO3 is in the form of ions and not the protonated acid.
Second one. HSCN
4.0 = 4.5 + log B/A
log B/A - 0.5 and B/A = 0.32 (check that and the HNO3 too).
Now you do the HCN and the HOCl and see what you get and you will be able to see which acids are more in the protonated form and which are not. I'll be glad to check your answers. You can see, however, that for HNO3 and HSCN that almost all of the acid is in the ionized form and not the unionized (the protonated) form. So the answer you're looking for must be HCN or HOCl or both.

Answer 2

Can you confirm that the answer is B

Answer 3

Answer is B on Apex; Both HCN and HOCl