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18. Aqueous Equilibrium

Whenever a **WEAK ACID** reacts with a **STRONG BASE** we use an **ICF Chart** to determine the pH of the solution.Â

Weak Acid-Strong Base Reactions

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So in the past we reacted a weak acid or a week base with water. And in these situations we used an ice chart. Remember, Icier stands for initial change equilibrium. And when we're utilizing a nice chart, we would use the units of mole Arat E. But now, in the situation, we're gonna react a weak acid with a strong base. And we need to realize that when react a week species like a weak acid with a strong species like your strong base, we're gonna have to not utilize a nice chart. But instead, in ICF chart here I cf stands for initial change final. And when we're utilizing an I C F chart, we're no longer gonna use molar ity. But instead we're gonna use moles when it comes to a acid based hi, Trish in the strong species must always be a reactant. In this case, the strong species is a base. It reacts and neutralizes an acid. The weak acid remember, an acid donates h plus. So what's going to happen here is the H plus will connect with O H minus to produce water. The n A plus left behind will connect with the N O to minus left behind to give a sodium nitrite sodium nitrate looks very close to the weak acid. Here, nitrous acid. It has one less h. So it is the conjugate base involved. This is the equation will utilize when discussing the different types of titrate shins involved in this weak acid strong base situation. Now that you've seen the basics, click on to the next video and see the different steps involved in this specific type of tight rations.

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when it comes to a typical acid based hydration, there are basically three places in which we discuss our calculations. Is it before the equivalents point at the equivalents point or after the equivalents point? Now, before the equivalence point has been reached, we would say that our weak species in this case are weak. Acid will be greater in amount than our strong species, which is our strong base. So, for example, if we have 0.250 moles off our nitrous acid are weak acid and let's say 0.100 moles of our strong base initially will say that we have zero of our products now. The difference here is will be looking at the react. Inside, the smaller moles will subtract from the larger moles. So what we have left at the end is zero of my strong base and 00. moles of my weak acid. And when it comes to the law of conservation of mass matters, neither created nor destroyed, it just changes forms. That means that if I lose 0.100 moles of my weak acid, it's not really lost, it gets transformed into moles of my conjugal base. So whatever we lose on the react inside, we gain on the product side here. When it comes to the water, we ignore it just like we would in an ice chart. And then what we have left at the end in terms of before the equivalence point. At the end, we'll have a weak acid remaining and conjugate base. Remember, when we have weak acid conjugate base, what does that mean? That means that we have a buffer. And if we have a buffer, we can utilize the Henderson Hasselbach equation, and that will help me find pH. So in this case, we would do pH equals P K, which is just the negative log of my weak acid nitrous acid. So you look up that k value plus log of the conjugate base, what's left of it, which is 0.100 moles, divided by 0.150 moles of my weak acid. And in that way we get our pH if we're dealing with the calculation before the equivalence point. So remember, when it comes to weak acid based height rations, when we're doing the calculations, it'll put us either before the equivalents point at the equivalents pointer. After the equivalents point in all three situations, we have to approach finding pH in a little bit of a different way. Now that we've seen this type of calculation, click on to the next video and see what happens after the equivalence point.

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So now we're taking a look at calculations after the equivalence point. Now, after the equivalents point, we'd say that are strong species is greater than our weak species. So in this example are strong base will be greater amount than our weak acid. So, for example, we're still starting out with 0.250 moles of my weak acid found. I would say we have 0.400 moles off my strong base here. We'll assume that we're starting out with zero of our conjugate base, but that's not always true. We're just going with zero initially for this example again, just like in a nice try, we ignore water. Now again, Look at the react inside. Look at the react. Inside, the smaller moles will subtract from the larger moles. So negative 0.250 minus 0.250 So at the end, we'll have zero of my weak acid, and we'll still have some strong based remaining 00.150 moles to be in fact, and we're gonna say again, remember the law of conservation of mass matters in the created nor destroyed? We lost 0.250 moles on the reacting side on Lee to gain it back on the product side. Now, At the end of this, what do we have remaining? Well, we have some strong base remaining and we have some contact base remaining. We no longer have a buffer because, remember, a buffer is weak acid and conjugate base. There is no more weak acid, so there is no longer a buffer. Therefore, we cannot use the Henderson Hasselbach equation. What's important here, though, is we have a strong species remaining, so we have strong base. At the end, the strong base will have a greater impact on the pH than a conjugal base. So Onley focus on the strong base. What we would do is we would take the moles left of the strong base and divided by the total leaders used within the question. We would use that information. Help us find the polarity of the strong base because you have a strong base. You could then take the negative log of that concentration to find p o. H. And once you have P o h, remember that pH equals 14 minus P o h. So again, we're at another spot in terms of our titrate shin. And again, you see that we're taking a very different process that we did from before the equivalents point. So just remember, here the strong bases, greater amount in the weak acid, so we'll have some strong base remaining at the end. With that information, we need to determine its polarity toe first, find the P. O. H. And then finally r ph click onto our last video to see what happens when we're at the equivalents point. What approaches need to be taken to find our Ph.

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in this third situation, we're now at the equivalence point At the equivalence point are weak species which is our week. Answered in this case is equal in terms of amount toe are strong species which is our strong base. So here would be the same exact moles as we have for the weak acid. And again we look at the react inside the smaller moles of track from the larger moles. But here they both have the same exact mall, so they completely destroy each other again. Remember conservation of mass. That means we're adding 2.250 moles to the product side. So at the end, all we have left is our conjugated base. So on Lee conjugate base will be remaining in terms of this situation. Conjugate base is equal to a week base. Remember, for weak acids and weak bases. In order to determine ph, we have to use a nice chart. So what we would do is we take the most left of our conjugate base and divided by the total leaders used in the question to find our polarity. We then set up an ice chart in order to determine our equilibrium expression because it's a week base, we'd utilize K B. Now, as we delve deeper into the situation of a weak, acid, strong based titrate Asian, we'll see examples that fit into these three situations. Right now, we're just laying down the groundwork for the three situations that arise, um, involving the Thai tray shin of a weak acid in a strong base. So just keep in mind what happens before the equivalents point after the equivalents point and at the equivalents point when these types of questions arise later on within our lesson.

Weak Acid Strong Base Titration Calculations

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So here it says, Consider the Thai trey shin of 75 MLS of 750.300 Mueller of H three c 303 with 12 miles of 120.450 Mueller K O. H. Here. We need to calculate the pH realize here that this is a weak acid based on the rules that we've learned about oxy acids, but also the fact that it has a k A less than one. So this is a weak acid. K O. H is a strong base. We have a weak species reacting with a strong species, so we know we need to do an I C. F chart. Remember, the strong species must always be reacted. So K o. H is a reactant. It is reacting with the weak acid because there's a strong species involved. We have a solid arrow going forward. Remember, what does the acid do? The acid donates in h plus and the base accepts it. As a result, we create the conjugate base que h two c 303 plus water. We have initial change final. Now remember, in a nice CF chart, we only care about three species we only care about the weak acid. We only care about its conjugate base. And we only care about the strong species. The fourth thing. The fourth compound, we ignore it. Next we need to use molds for a nice CF chart. Here were given volume of molar ity for both. Remember that mole Arat e equals moles over leaders. And if we multiply, both sides by leaders will see that moles equals leaders times molar ity. So what we need to do here is we need to convert these MLS in tow leaders and multiply them by the mole Aridjis to get moles. So divide your MLS by 1000 to get leaders, then multiply them by their mole Aridjis. So when we do that, we'll get these different moles. So we have 0.0 2 to 5 moles from my weak acid. And then for a strong base here. When I divide by 1000 mL, I get leaders multiplied by the molar ity that will give me my moles here for my strong base. We're not given any information on the conjugate base. So initially it zero now look at the react inside the smaller moles we'll subtract from the larger moles. So here are smaller moles which will subtract from my larger moles. I'll have left this much of my weak acid. I have zero left of my strong base. Remember, the conservation of mass will have the addition of this amount to the product side. So at the end of this tight, Trish in what do we have left? Well, at the end of this titrate shin, I have weak acid left and I have conjugate base left. What does this mean? This means that we have a buffer. So we are before the equivalents point, We have a buffer. So we use the Henderson Hasselbach equation. So ph equals P K, which is just the negative log. Okay. Plus log of conjugate base over weak acid. So can you get base over weak acid? Okay, so when you plug that into your calculator, negative log of the K gives me 2.82 point three 87 plus. Now, I take the log of everything in here. So when I do all that, that's gonna give me negative 0. So 2387 minus 501 gives me 1.89 as my final pH. So realize here we have a weak species with a strong species, so we need to set up a nice CF chart on the react inside the smaller moles of track from the larger moles. Look to see what we have left. We found that we had a buffer, so we used the Henderson Hasselbach equation to find our pH. Now that you've seen this example, take a look at the practice question that's left here. If you can approach it or don't know how to solve it, don't worry. Come back and see how I give the answer to this practice question. Good luck.

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now in order to create a buffer. 7.510 g of sodium cyanide is mixed with 100 ml of 1000.250 Mueller Hydro sonic acid. What is the pH of the buffer solution after the addition of 175 MLS off 1750.300? Mueller sodium hydride? All right, so hydro Santic acid represents a week binary acid. It has a name similar to sodium cyanide. This must represent the conjugate base. Now remember, the conjugate base has one less hydrogen than the acid form that hydrogen that it has, one less of has been replaced by a metal. So if hydro sonic acid is H. C N. Sodium cyanide is n a CNN, then we're gonna stay here. That sodium hydride here represents a strong base. That's because sodium Group won a medal connected to the hydride ion. Remember, if a group win a medal is connected to H minus O H minus and H two minus or O to minus, it will form a strong base. So we have a strong based species with two weeks species. Weak acid in contact base are both weak. Any time we have strong and weak species reacting. We need to set up an ice CF chart. Remember in the I C F chart, whatever its strong has to be a reactant. So the strong base has to be reacted because it is a base. It's going to react with its opposite. Its opposite is an asset, so Xcn would react with it. Now remember, what do acids do? Acids giveaway H plus So wolf the H puzzle connect with the H minus two. Give us hte too. Then the A C N will be formed in place as the conjugate base for one of the products. This year represents our chemical equation between the interaction amongst the strong base and the weak acid. Remember, since we have weak and strong, we have a nice CF chart in a nice CF chart. We use moles. First, we're going to realize that they gave us grams of our conjugate base. So we're gonna have to change that into moles. We have the formula for the conjugate base so we can figure out what the molecular weight of it ISS it has in it. One sodium, one carbon and one nitrogen. Looking up their masses on the periodic table. Sodium weighs 22.99 g. Carbon is 12.1 g and nitrogenous 14. g. When we write all those masses down, that gives us a combined mass of 49.1 g. So we're gonna take that 7.510 g of sodium cyanide divided by its molecular mass. And we'll know the moles of our conjugal base are most of concrete. Base comes out 2.1532 moles. So that's our beginning. Moles of our conjugate base Remember, uh, it's not always going to be that our products are zero initially. In this case, we actually have a beginning amount for our conjugal face. Now we're going to have to divide our milliliters by 1000 to get leaders and then multiply them by their mole Aridjis. Because remember, moles equals leaders times molar ity. So when we divide 100 miles by 1000 to get leaders, we multiplied by the 10000.250 Moeller That's gonna give me 0.25 moles of the weak acid divide 175 MLS by 1000 multiplied by its molar ity. And that gives me 0.525 moles next, remember? Look it on Lee the react inside the smaller moles will subtract from the larger moles. So minus 0.25 minus 0. This comes out of zero here. We're gonna still have some of our strong base remaining. Whatever happens on the react inside, the opposite happens on the product side because of the conservation of mass. So bring down everything at the end. What do we have left at the end? We have strong base remaining, and we have conjugal based remaining. We no longer have a buffer because the weak acid is completely gone. Remember, we have strong species left, so that means we're after the equivalence point. Also remember, if we have strong base and conjugate base, the strong base will have a bigger impact on the overall pH. So focus on the strong species. We're gonna figure out what its new concentration is, so we need to take its moles okay and divided by our total volume. What's our total volume? Well, we use the 100 miles of the weak acid and 175 miles of the strong base. Change those into leaders and add them together. And that gives us our total volume on the bottom. So that's gonna give me 0.100 Mueller, sodium hydride. Because we have a strong base, we confined P O h, which is negative log of this case off the strong base. So that's gonna give me negative log off 0.100 which comes out toe one. But remember, we don't want p o h. We want Ph. So here we say pH equals 14 minus ph. So that's 14 minus one. So our Ph you would be 13. So remember, if we have ah, a week species, any weak species reacting with strong species, we have to set up a nice CF chart. Whatever strong has to be reacted in this case is a strong base. It will react with its opposite. Its chemical opposite is an acid, so it has to react with a weak acid. Use that to help set up the initial equation. Once you do that, you can set up your I C f chart making sure the units are in moles. Remember, you look at the react inside the smaller moles subtract from the larger moles that will help you to determine what your final amounts of everything will be. In this case, we no longer have a buffer, but we have a strong species remaining. From that information. We can take the strong species and figure out its concentration. From there, you can figure out P O. H because it is a strong base. And once we know P. O. H, we confined pH. Remember these steps that are necessary for answering questions that deal with a weak acid and strong base after the equivalents point?

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considered the Thai trey shin of 75 MLS of 750.60 Moeller nitrous acid with 0.100 Mueller sodium hydroxide at the equivalence point will be the pH of the solution at the equivalence point were given. The K of nitrous acid is being 4.6 times 10 to the negative. Four. Now realize here that we have nitrous acid, which is a weak acid. We know it's a weak acid from the rules that we learn about oxy acids. We also know it's a weak acid because it's k A value is less than one. Here it reacts with sodium hydroxide, which is a strong base. So we have a weak species with a strong species reacting with one another. So we know we're gonna have to set up in I C F chart. Now here. We're going to say in our I c F chart, whatever strong has to be a reactant are strong base is going to react with weak acid. Okay, here we have an acid here we have a base. Remember, an acid donates H plus over to the base will produce H 20 and sodium nitrite. We have initial change final. Now remember, in a nice CF chart we have moles is the units. And because we're at the equivalence point, that means that my moles of acid equals my moles of base. Also remember that when it comes to moles, moles equals leaders times molar ity. Because of these two facts weaken say here that m acid times v acid equals and base times v base. So all we're doing here is we're adapting, adapting the dilution equation to help fit our acid and base definitions of being at the equivalence point. So here we're going to say we have 0.60 Moeller of our weak acid, 75 miles of it. We have 100 mol of our strong base. We don't know what the volume of the basis. So we divide both sides now by 0.100 Mueller. So that's gonna give me volume of the base equals 450 mL. Later on, we'll see why it was important to determine this volume for a base for now. Realize again, we're at the equivalence point. So whatever the moles of my acid are, my moles of base would be the same. Remember, we're gonna divide this these MLS here by 1000 and multiply them by the molar ity that will give us our moles of the nitrous acid. So when we do that, we get 0.45 moles again. Remember, we're at the equivalence point. So my moles of base are also 0.0.4 point 045 moles. We have no information on our conjugal based, so it's equal to zero. Remember, in an I c F chart, we only care about three things. The weak acid, the conjugate base and whatever is strong. We ignore this fourth species here. Now we look on the react inside, the smaller molds will subtract from the larger one. But here they're both the same amount of moles. So at the end, they're both going to be zero. Whatever happens on the react inside, the opposite happens on the product side. So we add this amount. At this point, we're gonna say all we have left at the end is conjugate base conjugate base is equal to the toe a week base. And remember, if you have a weak base or weak acid, how do we determine th we determine it with an ice chart. So at the equivalence point is the worst situation. We first have to set up a nice CF chart. Then we're gonna set up a nice chart. Now, remember, in a nice chart, there's a couple things we need to keep in mind. One units are in polarity, and secondly, we're going to ignore the metal off the conjugate base. That's because the metal involved is just a spectator ion. It's a neutral metal ion. So that means for ice chart. We're just gonna We're not gonna work with any, you know, to We're just gonna work with N O to minus. Okay, that's all we're gonna be concerned with. That's what we're gonna bring down into the ice chart. We need the polarity of this continent. Base similarity of the conjugate base will be the moles of it left divided by the total volume. So now we see why we had to determine the volume of our strong base because we need the total volume. So here we're gonna say that the total volume is the 75 MLS of the weak acid and the 450 m els of the strong base. So my total volume equals 0.75 leaders plus 0.450 leaders. All I did is I divided both the MLS by 1000. Okay, so that's my total volume. Take that total volume and plug it here on the bottom to find the polarity of my conjugal base. Okay, so at the end, that's gonna give me 0.857 Moeller of my nitrite ion. Now that we have the mole Arat e, we're gonna set up our ice chart. So remember, in a nice chart, are weak acid in our week based react with water. We have double arrows again because it's a week species water. He'll act as the acid. And this is my base, remember? And acid donates h plus, so we'll get H and l tube plus O H minus. We're dealing with initial change equilibrium. So remember, in a nice chart, we ignore water because the liquid our initial concentration, what we calculated is 0.857 Mueller products initially are both zero. We lose reactant in order to make products. Now we're dealing with the conjugate base. Remember, it's a week base week basis. Don't use K they use K B. So the K were given in the beginning, we're gonna have to change that two K b. So there's a lot of work involved in this. So we're gonna say, Remember K A Times K B equals K W. Plug in the number for K. Remember, kW is 1.0 times 10 to the negative 14. Divide both sides here by K A. So we'll get KB equal to 2.17 times 10 to the negative 11. So that's our KB that we're going to use. K B is equal to products. Overreact INTs, so it's equal to X squared over 0.857 minus X. Remember, we can use the 5% approximation method to see if we could ignore this minus X. So with the 5% approximation method, we're gonna take the initial concentration and divided by R K B. And if we get a value greater than 500 we can ignore that minus X. So the initial concentration of my week base is 0.857 Moeller, divided by the K B. We just calculated as 2.17 times 10 to the negative 11. That gives me 3.94 times 10 to the ninth. That number is much greater than 500. So that means we can ignore this minus X here, so we can drop this minus X and just solve for X now. So our K B look at our baby here, plug in the value that we determined we're gonna multiply both sides by 0. So that's gonna give you my X squared X squared equals 1.860 times 10 to the negative 12. Take the square root of both sides here. X equals 1.364 times 10 to the negative. Six Mohler. Now, remember, anytime we find X, that's gonna give us either H 30 plus or O H minus. If we look at our ice chart, the X that we just found is connected. Toa are h minus. Okay, so it's equal toe o h minus. So we're gonna say now p o. H equals negative Log of O. H minus concentration plug in the number that we just got. Okay, when we do that, it's going to give us our r p H So that's going to be r P O H 5. Because we know our p o h. We know our pH so pH equals 14 minus P o. H. So that comes out to 8. so we can see that there's a lot of work that goes into determining the pH at the equivalence point. Whenever we titrate a week species with a strong species again just to recap everything we went through, we realize here we're dealing with a weak acid in a strong base. So we know we need to set up a nice CF chart because we're at the equivalence point. They have equal moles. That means they're going to totally destroy each other in the reaction. And all we'll have left is conjugate base. Conjugate base is the same thing as weak base. So to determine its pH, we have to set up a nice chart. Once we set up the ice chart, we'll figure out what the concentration of O. H minus is. From that you can figure out your P o. H. And from there are pH. So keep in mind the steps necessary to determine the ph when a weak acid in strong base are reacting at the equivalents point, we set up a nice CF chart, then a nice chart.

Additional resources for Weak Acid Strong Base Titrations

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