 So this one says write a buffer reaction equation using a weak acid, weak acid and its conjugate base. Okay, so you're going to need to know your weak acids, but those are what you're going to memorize tonight. You're going to memorize the weak acids. Here are four weak acids that you could potentially use in this equation. In this case, since the practice exam uses the seeded acid, let's do a different one. Okay, let's do carbonic acid. So carbonic acid H2CO3, if it's dissolved in water, this is aqueous. If it's an acid base or with a weak acid that's a buffer, it's got to be an equilibrium. Because if it's not an equilibrium, then when you add hydroxide or protons into here, it won't go back and forth. Okay, so it has to be an equilibrium. That's why you need a weak acid. Okay, and so all you've got to really do is figure out, okay, I've got a weak acid. Here's my weak acid. I'm going to combine that with water. So now I've got to figure out, well, what's the conjugate base of this and what's the conjugate acid of this? Okay, so remember, this is your acid, this is your base here. So if we looked at this, right, the conjugate base, the conjugate base of this is going to be just this carbonic acid minus one of its protons, HCO3 minus. And then, of course, you're going to have to write the conjugate acid of your base, which is just the hydronium ion, H3O plus aqueous conjugate acid. That's why you have to do carbonic acid. This, in fact, is the buffer system that's used in our blow. The one thing I want you to be aware of is the negative charge here. Okay? How can I figure out that that's a negative charge? Okay? So what am I doing to this? I'm removing an H plus, right? So if I take H2CO3 and subtract an H plus from that, right? I'm going to, that's going to go to HCO3. Okay, that part probably is more obvious. But if you remove a plus sign, right, that means, so this thing's neutral, so if you're removing one of the positive charge, that means you're going to get a negative charge there. Okay? So just like with your water here, you're adding an H plus, right? You've got H2O plus H plus. So you're going to have H3O. Okay? That part's obvious. So you've got H, H2O, so it's going to be H3O, and you're adding a plus. You put your plus. That's where you get those charges on and come to get this. And come to get this.