Jules Bruno
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Now that we've seen this one, let's take a look at this example Here is say, determined the pH off a 0.50 Moeller NH four c l solution here, I tell you. The K B of N H three is 1.75 times 10 to the negative five. Now, technically, this is a salts question on what you should realize here is this could be in a mean, but what's messing it up? The chlorine at the end is messing it up. So that must be an indicator that we have assault. So we have NH four positive cl negative. We know that positive amines are acidic based on our solid ability rules or the acid base neutralization that we've learned. So we're going to say this is acidic and it's a weak acid. If you're acidic or your basic, you're going to react with water in a nice chart. So we're gonna take that NH four positive and reacted in a nice chart with water. Since it's an acid it's going to give away in H plus when it gives away that NH from that H plus becomes NH three, who's gonna accept that H plus water is going to accept that h plus to become h +30 plus. Then we're gonna stay here. Initial change. Equilibrium. Remember, Isis Initial change equilibrium, liquids and solids We ignore water is the liquid. So we're going to ignore it or they're gonna plug in the initial concentration 0.50 and H three and H 30 plus, we never talk about them. So there's zero initially. Now, remember, in a nice chart, we lose reactant to make product. Since we're losing reactant, this is going to be minus X. Since we're making product, this is gonna be plus X plus X bring down everything for the equilibrium line. So this is gonna be 0.50 minus X plus X plus X. Now we have to solve for X by using our k constant. Remember, this is an acid. Since it's an acid acids, use K A in a nice chart. But here we have K b. So you have to change your K B two k A. So we use the equation. K times k b equals K w. So we're gonna say here we have que, which we're looking for Times 1.75 times 10 to the negative five equals 1.0 times 10 to the negative 14. What we're gonna do now is we're going to divide both sides by 1.75 times 10 to the negative five. When we do that, our K equals 5.71 times 10 to the negative 10. We finally have our K. So now we can solve for X. So we're gonna say K equals products. Overreacting. It's so K, which is 5.71 times 10 to the negative 10 equals X squared because its products multiply, these two multiplying with each other. Give me X squared over 0.50 minus X. But remember, we can try to figure out if we can ignore that minus X. How do we do that? Well, we're gonna take the initial concentration and divided by the K value that we're using. And when this case we're using K. If you get a number greater than 500 a number greater than 500 then you could ignore that minus X. So if you took this initial concentration of 0.50 and divided it by the K, you'll get a number that's much bigger than 500. And because your answer is much greater than 500 you can ignore this minus X and avoid the quadratic formula. So again, when it comes to acid base ice charts just take the initial concentration and divided by the K value that you're using. This case, it was K A. If you get back an answer that's greater than 500 you can ignore that minus X. That means we assume that X is extremely small and really doesn't affect our concentration at all. So we can ignore. So let's just solve for X. Now we're gonna multiply both sides by 50. We're gonna say 5.71 times 10 to the negative. 10 times 100.50 gives me 2. times 10 to the negative 10 which still equals X squared. We don't want X. What we do want X, we don't want X squared, so we're gonna take the square root of both sides. When you do that, you're gonna get X equals 1.7 times 10 to the negative five. And remember, anytime we're dealing with acid base ice charts, actually there give me H 30 plus or O H minus. It's all based on which one is in your equation. In our equation, we have H 30 plus, which equals X. So this just gave me the concentration of H 30 plus and pH is just simply the negative log off H plus or H +30 plus. So take that number you just found and plug it in. When you do that, you're gonna get back an answer off 4. So that would be your pH. And I know acid based ice charts are a lot of work. And when you throw salt into the mix, it gets even messier. So you have to be very careful when you're doing questions like this. Identify the compound correctly to know if you have to do a nice charter. Not here. It broke up into NH four. Positive and see Oh minus. We know that this is a neutral ion because adding an H plus two, it creates HCL, which is a strong binary acid, which would mean that the negative ion is neutral because it's neutral. We don't have to show it at all in our in our ice chart, so we only focus on what's acidic or basic in our ice chart that gets plugged in. If you have an acidic ion, then it's gonna be a weak acid. If you have a basic I on that, it's going to be a week base. Remember, weak acid used K a week bases used K b knowing that will help you set it up correctly. If you're an acid, you're gonna give away an H plus tau water. If you're a base, you're gonna take away an H plus from water. This will help you figure out what your products are and help you figure out if X is going to give you a 30 plus or O H minus. Now, I know this first one was a little crazy with all the work, but now I want you guys to try to do the next practice question on your own. Follow the rules that we've looked at. Look at n a c N figure out. What kind of compound are you dealing with there? Once you figure that out, you know which way to go. And don't worry. If you get lost, come back and click on the explanation button. You don't see a video of me explaining how to approach this problem