 So, let's do one of these finding the Ka of a weak acid from the solution pH and the initial concentration of the weak acid, okay? So in order to do these types of problems, you're going to have to set up an ice level, okay? So let's read what the problem says. It says phenyl acetic acid, which is this thing here, and they give you the structure, builds up in the blood of a person with phenyl ketoneuria. An inherited disorder that if untreated causes mental retardation and death. A study of the acid shows that the pH of a 0.12 molar solution is 2.62, okay? What is the Ka of phenyl acetic acid? Is everybody okay with what we've written down so far? This equation wasn't given to you, okay? I wrote that down for you. You guys got to remember how to do these equations. So whenever you see the COOH, you know that's a weak organic acid, so it will behave like this, okay? Is everybody okay with it? Okay, wonderful. So, now what we can do is we can perform an ice table or make an ice table here, okay? So, ice and of course in our ice tables, liquid, we don't worry about. The initial concentration of our acid was 0.12, like that. The initial concentration of these was nothing, okay? They didn't tell us that they had any of those things in them, okay? So we're going to put zero here and zero here. So on the reactant side, what happens? We're going to subtract stuff, right guys? So what's going to be here? Minus x, right? And here? Plus x and here? Plus x as well. So what's going to be the equilibrium? Remember the E is equilibrium concentration is going to be 0.12 molar minus x for that, right? Here we're going to be x and here we're going to be x. Is everybody okay with doing that so far? Okay, so kA, it wants us to figure out, well, what is the kA? Well, what is the equation for the kA? Let's figure that out first. So the kA equation is going to be what? So the conjugate base here, right? The concentration of that times the concentration of the hydronium line. Is everybody okay with that, you guys remember? You guys remember about this one? All of you guys, C6H5, CH2, COO, like that. Times 3O1, concentration like that, that makes sense. Okay, and then down here what are we going to have? Okay, so what do we know about this? Normally we would have to use the quadratic formula here, okay? But we're going to invoke the 5% rule, okay? So instead of having 0.12 minus x, because we know this is a very weak acid, so kA is not going to go to that way for any appreciable extent, okay? So we can effectively erase that like that. That's going to be our equilibrium concentration. Is everybody okay with me doing that? So what we're going to do now is, well, we know some other things. We know the pH of this solution, right? So let's figure out what the H3O concentration is. So what is that? How do we figure that out from the pH? 10 to the negative 2.62, right? So let's figure that out. My calculator is 4 times 10 to the negative 3 molar, okay? So that's the pH at equilibrium. So that, not the pH, that's the hydronium ion concentration at equilibrium. So notice that's x, right? So that means that this also is that concentration there. Does that make sense to everybody? So if we, let's write those two things down really quick. So let's do the 5% rule right now just to justify what we're doing, okay? So remember the 5% rule is going to be this divided by the initial concentration. So x minus the initial concentration times 100%, all right? So x divided by the initial concentration. So is it okay that we use this assumption? Yes, so if this is greater than 5% then we can do that. But since it isn't, we can do this. Is everybody okay with that? That's wonderful. So I'm going to erase that portion. Can we do it? Here, I'll let you, you got it? Okay, you got it? Okay, so now all we do is really plug and check, right? So it's going to be this is that. So 2.4 times 10 to the negative 3, 2.4 times 10 to the negative 3, divided by, and notice I'm not putting my molars in there, okay? Divided by 0.12. So imagine for a weak acid, it should be very small, right? Okay, so I got 4.8 times 10 to the negative 5. That's the k-h for phenyl acetyl acid. Does everybody else get this number? Yes. If you type it up. Got it? Okay, wonderful. Good job, guys. Are there any questions on this one before we do it again? I need to do h3o, like, how'd you get 2.4 times 10 to the negative 3? Well, because I figured out the pH over there. So I just don't get how you plug it in under the C6H5C. Well, okay, so that's a good question. What is x equal? Is x equal to x? Yeah, so x equals x. So those two things equal each other, okay? So it's just like 1 equals 1, okay? So that's how you do it. The only weird thing, and it's really not weird, because remember we talked about the 5% rule is erasing that other thing, you know? But if you can try, in fact, it's a good exercise to plug x in for that and see if you get a different answer than this. I would be willing to bet that you don't get a different answer to two significant things. Okay, but it's a good question. Okay, any other questions about that or anything? Okay, wonderful.