Both H2O and H2PO4- are amphoteric. Write an equation to show how each substance can act as an acid and another equation to show how each can act as a base.

Video transcript

Welcome back, everybody. Our next problem says both water, H2O and H two P four minus are amphoteric. Write an equation to show how each substance can act as an acid. And another equation to show how each can act as a base. So let's recall what amphoteric species are as we can recall based on this problem saying to show how they can act as an acid or a base. And amphoteric species is a substance that acts as both acid and base depending on its environment. And one sort of hint or tip that we see is that most amphoteric substances we see start with an H at the beginning and end with a negative charge. So in H two po four, we have our hydrogen, in fact, two hydrogens and then at the beginning and a negative charge at the end. And an obvious exception to this is water right here, which does not have a negative charge at the end, but does act as an amphoteric substance. So let's write our acquaintance. So we'll start with H two P four minus and of course, it's an aqueous solution and we'll have it react with water which is liquid and let's have it behave as an acid in this case. So I'm going to note acid over my H two po four minus. So we'll have an equilibrium, of course. And then recall that an acid is a substance in this context that donates a proton. So we're going to get rid of one of those hydrogens and a positive charge. So our conjugate base in this situation will be H just H we've lost one of our hydrogens po four. But now we have, we've lost a positive charge. So now our negative charge is not just a minus one but two minus HPO 42 minus. And this is of course aqueous. And then as our H two po four minus has lost a proton, the water has gained it. So instead of water, we now have H 30 plus or our hydro num ion in an aqueous environment. So in this case, we have H two po four minus acting as an acid if it's acting as an acid, what is the water doing? The water is acting as a base. So here's our H two po four T minus as an acid. Water is a base. So now let's just make the reverse happen. Well, we know, so we'll start again, H two po four minus and now we want to act as a base. So it's aqueous and it will react again with water. And now, if our water is acting as base before our water is now going to act as the acid. So again, our base will be a substance that accepts NH plus. So we add a proton onto this. So now we have three hydrogen. So we have H three and then no more negative charge, it's now neutral. So H three po four will be our conjugate acid there. And then the water which is going to act as an acid and donate an H plus. It's add aqueous here to our is now going to be oh minus. So a hydro I am having lost a proton. So there we have it, we have our water and our H two P four minus acting in the first case as acid and in the water as a base. And in our second equation, our dihydrate phosphate is acting as a base and water is acting as an acid. See you in the next video.