Acid strength-HClO4 vs H3PO4 (HW 12.51d)

[Izzy Ick 4B]

HW 12.51 part d

When you compare acids like HClO₄ and H₃PO₄ that differ in multiple ways (number of H atoms, Cl v. P), how do you determine which one is stronger?

[Jonathan Shih 3H]

In this case, I think you just have to know that perchloric acid is a strong acid so it dissociates completely in an aqueous solution while phosporic acid simply does not as it is a weak acid. Sorry if I can't come up with a better explanation but I think this one of those more black and white answers where you just have to memorize the list of strong acids!
Hope I helped!

[Chem_Mod]

To approach this problem, we should look at the similarities of the polyatomic ions: there is one central atom (phosphorus/chlorine) and 4 surrounding oxygen atoms to which hydrogen atoms will attach. Chlorine has a higher electronegativity than phosphorus, so it pulls the oxygen atom electrons towards itself with greater force than phosphorus. The fact that chlorine has a higher pull means that oxygen electrons become less accessible for the hydrogen atoms to "share", making it less favorable for hydrogen to stay bonded versus dissociating from the acid. Phosphorus by contrast pulls the electrons less, meaning that hydrogen has greater access to oxygen electrons, making it less favorable for it to dissociate, hence why H₃PO₄ is a weaker acid.