how do i tell if a substance is amphiprotic just from the equations?
You need to look for something that acts as both an acid (proton donor) and a base (proton acceptor). So, let's go through the options one by one:
1. H2PO4-: This only appears once in any of the equations, as an acid (first equation). Since it doesn't act like a base, it's "not" amphiprotic (I don't like this question, because H2PO4- IS a base, but doesn't act like a base in the equations, so they want you to assume it's not. Dodgy questions 101)
2. NH4+: This, once again, only appears once, which is as an acid in the first equation. Since it doesn't act like a base, it's not amphiprotic.
3. CO3 2-:Once again, it only appears once! It's acting as a base this time (second equation), but not an acid, so it's not amphiprotic.
4. HPO4 2-: Okay, this one actually appears twice! In the first equation, it gains a proton, so it's acting like a base. In the third equation, it donates a proton, acting like an acid. So, HPO4 2- is our amphiprotic species.
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