Which of the following is the strongest acid?

[HealthHare]

---

Members do not see this ad.

From EK gen chem 1001
Q810

A. Mg3(PO4)2
B. CaHPO4
C. K2HPO4
D. NaH2PO4

I said A since I thought you just had to compare the cations and since B-C are neutral, A would be the strongest acid. But EK says D is the answer since H3PO4 loses protons.

I don't understand this answer choice.

 

[Brunerm]

If you were purely looking at the cations, then you would be correct. Mg2+ is more acidic (pH~5) than Ca2+, Na+, and K+ (these three cations have a pH~7).

However, the anions are what you should be looking at because they are your more common acids/bases. It is good if you come acquainted with this table here. This is the best I could find right away, but a better one would have the Ka values (higher Ka=more acidic). So H3PO4 is a triprotic acid (3 H+'s) and can dissociate twice to form H2PO4- and HPO4 2-, which are amphoteric (both act as an acid and a base). HPO4 2- will dissociate once more to form PO4 3-, which is a base. Now to determine which acid is the strongest. A multiprotic acid's Ka decreases as it loses an H+. Therefore,

Ka of H3PO4 > Ka of H2PO4- > Ka of HPO4 2-

or

Ka1>Ka2>Ka3

Based off of this info, you know that choice A is basic and choice D is more acidic than choice B & C.

 

[mcat prep]

An alternative approach . . .

The AAMC wants you to know that an important definition of an acid, is as a proton donor (Brønsted-Lowry).

All things being equal (i.e. among phosphates OR among sulfates, etc.), the strongest acid has the most dissociable protons.

Defining acids and bases reflexively according to Brønsted-Lowry will get you many easy marks on the real exam in both gen and org chem (of course, you should be familiar with the Lewis definition which comes up more rarely).

 

[HealthHare]

Thank you both for making this much clearer.