 Okay, so let's try this one now. It says calculate the hydroxide ion concentration of a solution with a pH of 4.0. Okay, so we're looking for the hydroxide ion concentration, but pH gives us the hydronium ion concentration or the proton concentration, right? So there is something else we know, though. We know kW. kW is the concentration of H plus or H 3 O plus times the concentration of OH minus. And we also know that kW is a value of 1.0 times 10 to the negative 14. So with that information, we should be able to rearrange this to, after figuring out this, being able to plug that number into here. Okay, so let's figure out what the hydrogen ion concentration is first. Okay, so how do we do that? Well, the same way we did it the last problem, right? So it's going to be 10 to the negative pH equals the H 3 O plus concentration, is that right? So in this case, why don't we just add a zero there? So we can have two sigthins. So, well, we don't need to do that. I don't need to do that. What is the answer there? 1.0. Tell me how, guys. Thanks. 10 to the negative 4. What? I'm all right. Okay, so if it's right on the dot, it's always going to be that to the negative 4 or whatever. So that's the hydronium ion concentration, but we're looking for the hydroxide ion concentration. We know KW, we know hydronium ion, so the hydroxide ion concentration, we just divide both sides by H 3 O plus, right? So we're going to get KW divided by H 3 O plus concentration. Do we know both of these numbers? Are you excited that we're going to get KW divided by the negative 4? Do you guys remember how to do that? Yeah. So again, you don't have to, right? Put these all into your calculator if you remember your law rules, right? So if it's negative on the bottom, you're just going to bring it up to the top and add it. So it's going to be, what did you guys say? 1.0 to the time it stands to the negative 10 mole. So this is a negative on the bottom, so if you do that, you bring it up to the top and put a plus. So if you do minus 14 plus 4, it's going to be negative. If they were together, if they were both on top, then you would add them together. Does that make sense? Okay. So again, there's that sheet that I printed out for you guys, the law rules, okay? It makes things real easy on you if they're, especially if they're just 1.0 times whatever, you know? Because they're straight up, you can move them around. It's real easy. Okay, is everybody cool with this? Any questions? Do you kill it?