Determining Predominate Species - Video Tutorials & Practice Problems
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Determining Predominate Species
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In this video, we're gonna talk about how to determine the predominant species of a molecule. So in our last lesson video, we reviewed the Henderson Hasselbalch equation from your previous chemistry courses, and we know that the Henderson Hasselbalch equation reveals the ratio of the concentration of conjugate base to the concentration of conjugate acid. And it turns out that the conjugate base and the conjugate acid are actually different forms or species of a molecule. And so the predominant species is really just referring to the most abundant form of a molecule that exists under a specific set of conditions. And so it turns out that the pH of the solution, as well as the pKa of the acid, will determine or dictate the predominant species of that acidic molecule. And so in our next lesson video, we're going to compare the pH of the solution and the pKa's of acids to determine the predominant species of those acidic molecules. So I'll see you guys in that lesson video.
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Determining Predominate Species
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Video transcript
So we can determine the predominant species of a molecule by comparing the solution pH to an acid's pka, which will reveal the relative concentrations of the conjugate base, as well as the concentration of the conjugate acid. And so recall that the conjugate bases are deprotonated molecules because they have one less hydrogen than the conjugate acid. And the conjugate acids, of course, are going to be protonated molecules, and that's because they have one more hydrogen than the conjugate base. And so down below in our example, we're going to compare the pH and the pKa to fill in all of the blanks in our chart below. And so when we say compare the pH to the pKa, what we really mean is compare the value of the pH to the value of the pKa and determine if those two values are either equal to each other or if one value is greater than or less than the other. And so, one way that helps me determine the predominant species is by always associating the a in the pKa to the a in the concentration of conjugate acid. And so by always associating the pKa with the concentration of conjugate acid, I'm able to quickly determine the predominant species. So let me show you what I mean. So when the pH is equal to the p k a, I know that the concentration of conjugate base is going to be equal to the concentration of conjugate acid. So pretty easy. Right? When the pH is less than the p k a, because I'm associating the pKa with the concentration of conjugate acid, I know that the concentration of conjugate base is going to be less than the concentration of conjugate acid. So essentially, it's going to have the same relationship as the pH over p k a. And so when the pH is greater than the p k a, again, because I'm associating p k a with the concentration of conjugate acid, I know that the concentration of conjugate base will be greater than the concentration of conjugate acid. And so that's pretty easy, right? Associate the pKa with the conjugate acid, and you'll be able to quickly determine the predominant species. And again, the predominant species is going to be the one that's in highest abundance. Now, what you'll notice here is when the pH is equal to the pKa, the concentration of conjugate base is exactly equal to the concentration of conjugate acid, which means that there is no predominant species since since neither is in highest abundance. They're both equal to each other. And that means that 50% of our molecules are going to be in a deprotonated conjugate base form, and the other 50% of our molecules are gonna be in a protonated conjugate acid form. Now, down here when the pH is less than the pKa, the conjugate acid here is in higher concentration, which means the conjugate acid is predominating. And if the conjugate acid predominates, we know that the majority of the molecules are going to be protonated, since our conjugate acid here is always protonated, like we said up above. Now, when the pH is greater than the p k a, the conjugate base here is in higher concentration, so the conjugate base predominates. And we know that the conjugate base is a deprotonated form, so that means that the majority of the molecules are going to be deprotonated. And so in our final column over here on the far right, notice we have the Henderson Hasselbalch equation, which we know is the pH is equal to the pKa of the acid plus the log of the final concentration of the conjugate base over the final concentration of the conjugate acid. Now what you'll notice here is that we have the same exact equation for all three of these different scenarios, and the only thing that's gonna be different is this ratio of the concentration of conjugate base to the concentration of conjugate acid. Now when the pH is equal to the pKa, we know that the concentration of conjugate base here, which I'll abbreviate with CB, is going to be relatively the conjugate acid, which I'll abbreviate with CA. Now when the pH is less than the pKa, we know that the conjugate acid is predominating, and most of the molecules are going to be protonated. So that means that we're gonna have a large concentration of conjugate acid, and on the other hand, we're gonna have a much smaller concentration of conjugate base. So we can draw it in small here to represent the smaller concentration. Now, with this last scenario, when the pH is greater than the pKa, because we know that the conjugate base is gonna be predominating and most of the molecules are gonna be deprotonated, that means that we're gonna have a large concentration of conjugate base, and a smaller concentration of conjugate acid. And so the sizes of these, ratios that we've drawn here indicate the approximate concentrations. So essentially, what we're saying here is, these sizes are representing what we see in this column over here. And so moving forward, we're going to be utilizing a lot of these concepts, and it's important for you guys to take some time to study this chart and to commit some of it to your understanding. So, rewatch this video a couple of times, and we'll be able to get some practice moving forward in our next couple of practice videos. So I'll see you guys there.
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Problem
Problem
Consider the following pKa value for pyruvic acid. Which of the following species predominates at pH = 7.4?