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I am trying to figure out if a drug that I have is acid or base and strong or weak. On drugbank database, I found the following pKa. I have been reading and understand that higher the pKa stronger the acid (and weaker the base) and vice versa, however, based on two reported pKa values, how do you determine if it is a base or acid and strong or weak?

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  • $\begingroup$ The lower the pKa of an ionizable group the more likely it is to dissociate an H+ ion (proton) thus the more acidic the group is. pKa is nothing more than the negative logarithm of the Ka which is the acid dissociation constant. Determining whether a group is an acid or base is more difficult, because this depends on a couple of factors such as the inductive effect caused by other atoms in the molecule. There are quite some tables online of the most functional groups with their respective pKa's, but in general carboxyl groups are weak acids and amines weak bases. $\endgroup$ – user21398 Mar 16 '17 at 23:33
  • $\begingroup$ Hello and welcome to Chemistry.SE. If you have any questions about the site a good place to start is with the short tour. You can also visit the help center. I see you've already gotten some feedback on you question. Good luck with it, and enjoy! $\endgroup$ – airhuff Mar 16 '17 at 23:59
  • $\begingroup$ First value is "normal" pKa, second is pKa of conjugated acid. $\endgroup$ – Mithoron Mar 17 '17 at 0:08
  • $\begingroup$ Remember pKa is the -log of the Ka and if you can remember the relationship between Ka and how acidic something is then you'll be set. $\endgroup$ – Tallon Watkins Mar 17 '17 at 1:40
  • $\begingroup$ thanks everyone for clarification. Your answers are very helpful. $\endgroup$ – Krina M Mar 17 '17 at 3:14
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Sorry, you have misunderstood the concept of acidity. Low $\mathrm{p}K_\mathrm{a}$ means stronger acids. High $\mathrm{p}K_\mathrm{a}$ means weaker acids.

I will give you two examples. Acetic acid has $\mathrm{p}K_\mathrm{a} = 4.76$. Hydrochloric acid has $\mathrm{p}K_\mathrm{a}$ of about $-7$. Surely, hydrochloric acid is a much stronger acid than acetic acid.

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$K_\mathrm a$ refers to the degree of dissociation of the acid.

Consider: $$\ce{HC2H3O2(aq)<=> H+(aq) + C2H3O2-(aq)}$$ $$K_\mathrm a = \frac{[\ce{C2H3O2-}][\ce{H+}]}{[\ce{HC2H3O2}]}$$

If the acid dissociates to a large extent, i.e. high $[\ce{H+}]$ ions, then its $K_\mathrm a$ will be high. When you convert a high $K_\mathrm a$ to $\mathrm p K_\mathrm a$, $\mathrm p K_\mathrm a$ will be low (just a mathematical relationship).

Thus, high $K_\mathrm a$ = low $\mathrm p K_\mathrm a$ = strong acid. High $K_\mathrm b$ = low $\mathrm p K_\mathrm b$ = strong base.

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