Jules Bruno
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So let's take a look at this first question here. It says Paradyne and organic molecule is a very common, weak base, and we should know that it's a week base because if we take a look at purity, it has carbon, hydrogen and nitrogen. Remember, that's an indication that we have in a mean it doesn't have a positive charge on it. So it's a neutral mean, which would make it a week base again. The rules are incredibly important to remember, and in this question I make it really easy for us. I tell you, it's a week base, but expect your professor is not to be that obvious about it. They'll give you a compound, and it's up to you to identify it as either an acid or base, weak or strong. All right, so we have a weak base here. It says assume you have a 0.2 to 5 Moeller solution of purity determine its pH. The K B value for the compound is 1.5 times 10 to the negative nine. Now, we're gonna need a lot of room toe work this first ice chart out, so I'm gonna remove myself from the image guys. So we have more room to work with. All right, so we know we have a weak base. And I told you have said it multiple times. If you have a weak acid or weak base, we have to use a nice chart. So let's set up this ice chart. We're gonna say C five h five n plus water. So I'm just rewriting the equation we have above. And we know this has to be the equation because remember, if this is the base, then the water has to be the acid. So the water has to give away an H positive to the base. The base accepts the h positive, and that's how it became this conjugate acid. The water loses an h positive, and that's how it becomes this conjugate base. Now we're gonna say we're gonna do a nice charts. So its initial change equilibrium. That's what I stands for. And remember, in a nice chart, we ignore two phases. We ignore solids and liquids. The water is a liquid, so we're going to ignore it. So again we ignore solids and liquids. Next, we're gonna say, since this is a nice chart, dealing with acids and bases. We have to have the units in Mol Arat E. The good thing is already gave you the units and it is more hilarity. So if they didn't give it to you and polarity, you have to make sure you convert them to that. I don't tell you anything initially about the products, so assume they're zero. Now, here's the important thing. The important sentence you need to remember for acid and base reactions. We lose react INTs to make products. So we're losing the reacting. So it's gonna be negative X because we don't know what the changes remember, it's weak. We don't really know how much of it is breaking down. And if we don't know how much of it's breaking down, we don't know how much product we're making. So this is gonna be plus X because their products were making them bring down everything. So this is gonna be 0.2 to 5 minus x plus X plus x. So we've just filled out our first ice chart for a week base. Now what we're gonna say is, since it's a week, base bases used K b. So here we're gonna say K B equals products. Overreact. It's. And I told you the value right here. It's 15 times 10 to the negative nine. So let's just plug in what we know. This will be 1.5 times 10 to the negative nine, and then we're gonna have the products multiplying. Just write the equilibrium expression here. Okay, So this is the equilibrium expression we have, and these two things are X in here and in here. So x Times X is X squared, divided by 0.2 to 5 minus X. Now, here's a crappy thing about ice charts. Sometimes we have to use the quadratic. The good thing is, when it's acid and base chemistry, we can use a shortcut to see if we can ignore this minus X. If we could ignore that minus X, we don't have to do the quadratic. So to figure out if we could ignore or not, we're just simply going to do this. We're gonna take our initial concentration. Our initial concentration is this number and divided by r k value. Whichever que were using. In this case, we happen to be using K B. So the over KB. But if we use another if we use another question in the value have to be K a. Then it will be the initial divided by K. We're gonna say if you get an answer greater than 500 then you can ignore the minus X. So just remember that this is the trick we're gonna use to see if we could ignore that minus X, take the initial concentration divided by the K value were using. If you get an answer back 500 greater than 500 that means we could ignore that minus X, because that minus sex is gonna be so small it doesn't affect our real answer. So we're gonna take the initial concentration, which is 0.225 and divided by the K value 1.5 times 10 to the negative nine. When we do that, we get 1.5 times 10 to the seventh. So that answer is incredibly large, much bigger than 500. Which would mean we can ignore this minus X. So this is gonna make our math a lot easier, because now we don't have to do the quadratic, so we're gonna do here is gonna multiply both sides by 0. So we're gonna say now X squared equals 3.375 times 10 to the negative 11. We don't want X squared. We just want X. So take the square root. So X here equals 5.81 times 10 to the negative six. Now, when it comes to acid base chemistry, when you find X X gives you either oh h minus or H 30 plus And it's all based on which one is in your equation. In our equation, we have h plus and at equilibrium H plus equals X. So we just found Here is the concentration of O H minus. And if we know the concentration of O H minus, we know the p o. H. Because remember, p o H equals negative log of O H minus. So plug it in. So here we're gonna say here that means that r p o H equals 5.24 But I didn't ask for p. O. H asked for ph. So now you're gonna say ph plus p o h equals pH equals 14 minus P. O h So what equals 14 minus 5. So your answer here would be Now, I know it looks like a lot of work, and it iss and you have to be very careful with all the math that you're doing. One little mistake could greatly affect your answer, but this is the way we would set up our ice chart. Now there's gonna be a few variations of different types of questions they can ask. But don't worry. The other videos will cover different varieties of ice charts. This is just our first one. Hopefully, you guys didn't get lost along the way as we did this. If you got a little confused here and there, feel free. If you have to post a question dealing with it. If you have any questions on this one, or just go back and look at the video again, we know we had a week based, so we had to do this. The thing is week basis produce always minus as a product. So we have to change our O H minus into P O. H. And then from P O H. Two ph. If we started out with a weak acid. It will produce H 30 plus we find X, and that will give us a three plus and then you simply to find Ph. Take the negative log of H 30 plus and that will give us pH. Hopefully, guys, you guys get really good at thes because it does take practice, so make sure you pay attention to all the steps that we're doing.