Diprotic & Polyprotic Buffers

by Jules Bruno
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here, it says. Calculate the pH of 100 miles of a 1000.25 Mueller carbonic acid solution when. 70 miles of 700.25 Mueller sodium hydroxide er Added here carbonic acid is die protic acid, so it has to K values. Now the challenge with these types of questions is understanding which forms of the die protic acid or Polly protic acid are involved in the question. So remember carbonic acid H two co three. This is it's acidic form when it still has both of its acidic hydrogen. When it loses its first hydrogen, it becomes H seal three minus, which is hydrogen carbonate or bicarbonate. Then, when it loses its last acidic hydrogen, it forms carbonate ion. We have to determine in this question which one of these forms are involved, and what we need to realize here is that carbonic acid, because it's K values, are less than one. It represents a weak acid, which I'll abbreviate is W. A. Any weight we should know at this point represents a strong base. Now, remember what I've said in the past. If you have a weak species reacting with a strong species, then that means you're going to have to set up in I c f chart. But again, we still have to figure out which versions are reacting here. So we're gonna say that we have to figure out basically the equivalent volume of the strong species needed to reach the equivalence points for this die protic acid. All right, so we're gonna say here M acid times V acid equals M base times V base. And all we're gonna do here is we're gonna calculate how much of the strong species do I need to add in order to get to my first equivalents point? So my acid here it's polarity is 0.25 Moeller. Its volume is 100. Emmaus. The polarity of my strong base here is 0.25 Moeller. Now, I know I give us a volume here, but we're gonna ignore that volume for right now. What I'm trying to determine is how much of the strong species in milliliters do any toe add to get to the first equivalents point of carbonic acid. So divide both sides now by 25 Mueller and you'll see here that the volume of my base needed to get to the first equivalents point is 100 m. Els. This is important, okay? Because we need 100 miles to get to the first equivalents point for carbonic acid. And we're not there were only using 70 m. Els. Okay. And so the way you look at it is this You're gonna say that we use these two forms before we reach the first equivalents point point volume. Okay. So, again, what you have to do when it comes to die product and Polly product acids when they're reacting with the strong species is determined. What is the equivalent volume needed of these strong species to get to the first equivalents point? If we haven't reached the first equivalents point volume yet, then we use these first two versions. Now, if our volume of the strong base had been greater than 100 Emmaus, then that would mean that we passed the first equivalents point. So we're gonna say here these other two forms, um, this is after we have reached Well, this is after we have passed the first equivalents point volume. That's when we use the other two forms. So, again, we needed 100 mls of strong base. We only have 70 MLS. So it's not enough to get us beyond the first equivalents point. So that means we're gonna use the first two forms of carbonic acid, Theus acidic form and its intermediate form. So we're gonna bring down H two co three, which is the weak acid form. Remember, the strong species must always be a reactant. So my bases the strong species, it reacts with the acid. That's why I drew the weak acid form with it. Because in a way, which is a strong species, we're gonna have a single arrow going forward. Remember, when we have weak and strong together, we use an I C F chart. So initial change final here. This is my acid. This is my base acids donate h plus. So the carbonic acids gonna donate an H plus to the minus. That's gonna produce water. And then we're also gonna have the n A plus, combining with the three minus left behind. Remember, in an I C F chart, we only care about three things. We only care about what is strong. We only care about the weak acid. We only care about its conjugate base. The fourth thing, we're gonna ignore it. Also remember that in an I C f chart, the units have to be in moles. Moles equals leaders, times more clarity. So divide the MLS by 1000 to get leaders and multiplied by the mole Aridjis. So when I divide the the MLS 5000 get leaders and multiply them by their polarities, I'm going to get 0. moles of carbonic acid and 0.175 moles of sodium hydroxide. We don't have any of the conjugate base, so that zero initially remember Look at the react Inside, the smaller moles will subtract from the larger moles and whatever happens to the react inside, the opposite happens to the product side. So this will be plus 0.1 point +0175 Moles bring down everything so would get 0.75 moles. Zero here 0.0 175 moles. We look to see what we have at the end of our reaction. At the end we have weak acid left and at the end we have conjugate base left. So remember, if you have weak acid and conjugate base left at the end, you have a buffer. Which means I get to use the Henderson household back equation. So Ph equals p. K. And we have to determine which PK so remember, if we're dealing with the acidic form and its intermediate form, that means they're dealing with K A one k one is what connects them together. If we're dealing with the intermediate form and the basic form, that means we're dealing with K two. All right, so in this case, we're dealing with the acidic four minutes, the intermediate form. So that means we're dealing with K one, which would mean that this is PK one plus log off conjugate base over weak acid. Alright, So PK one that will be negative log of K one, which we're told this 4.3 times 10 to the negative seven plus log off the moles of my conjugate base divided by the moles of my weak acid. If you do it correctly, you'll get Ph equals 6. Okay, so you just take the negative log of this value here, take the negative log of these guys here, and then add those two answers together. So again, dealing with buffer questions. When it's dealing with a model product, acid can be challenging. But the difficulty of the questions increases when you're dealing with di product and Polly product acids. So again, you have to make sure. At what point are these tight rations occurring? Are they occurring before the equivalents point? Are they after the first equivalence point? Are they occurring after the first equivalents point that determines which versions are being used in the question and then also determines which peek A you'd have to use in the question. So as we continue with these discussions, make sure you take note of all the little things we talked about as we approach problems like this.