Determining Predominate Species - Video Tutorials & Practice Problems
On a tight schedule?
Get a 10 bullets summary of the topic
1
concept
Determining Predominate Species
Video duration:
1m
Play a video:
in this video, we're going to talk about how to determine the predominant species of a molecule. So in our last lesson video, we reviewed the Henderson Hustle back equation from your previous chemistry courses. And we know that the Henderson Hustle back 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 exist under a specific set of conditions. And so it turns out that the pH of the solution, as well as the P k a 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 P. K s of acids to determine the predominant species of those acidic molecules. So I'll see you guys net Listen, video
2
concept
Determining Predominate Species
Video duration:
5m
Play a video:
so we can determine the predominant species of a molecule by comparing the solution Ph two and acids PK, 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 base is R D protein ated molecules because they have one less hydrogen than the conjugate acid and the conjugate acids. Of course, they're going to be protein ated molecules, and that's because they have one mawr hydrogen than the conjugate base and so down below. In our example, we're going to compare the pH in the PK to fill in all of the blanks in our chart below. And so when we say, compare the Ph to the peak A. What we really mean is. Compare the value of the pH to the value of the PKK 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 peak A to the A in the concentration of conjugate acid. And so by always associating the peak A with the concentration of conjugate acid, I'm ableto quickly determine the predominant species. So let me show you what I mean. So when the P H is equal to the P K, 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 peak. A. Because I'm associating the PKK 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 gonna have the same relationship as the pH over PK. And so when the pH is greater than the PK again, because I'm associating PK 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 P k with the conjugate acid and you'll be able to quickly determine the predominant species. And again, the predominant species is gonna be the one that's in highest abundance Now, which will notice here is when the P H is equal to the PK. The concentration of conjugate base is exactly equal to the concentration of conjugate acid, which means that there is no predominant species. Since neither is in highest abundance, they're both equal to each other. And that means that 50% of our molecules are gonna be in a deep protein ated conjugate base form, and the other 50% of our molecules are gonna be in a protein ated conjugate acid. For now, uh, down here, when the pH is less than the PK, the conjugate acid here is in higher concentration, which means the conjugate acid is predominate ing. And if the conjugate acid predominates, we know that the majority of the molecules are going to be protein ated. Since our conjugate acid here is always protein ate it like we set up above. Now, when the pH is greater than the PK, the conjugate base here is in higher concentrations. So the conjugate base predominates, and we know that the conjugate base is a deep protein ated form. So that means that the majority of the molecules are going to be de protein ated. And so, in our final column over here on the far right notice we have the Henderson Hustle back equation, which we know is the P H is equal to the P K 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 based to the concentration of conjugate acid. Now, when the P H is equal to the PKK, we know that the concentration of conjugate base here, which I'll abbreviate with C B, is going to be relatively the same size and magnitude as the conjugate acid, which I'll abbreviate with C A. Now, when the pH is less than the PK, we know that the conjugate acid is predominate ing and most of the molecules are going to be pro nated. 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 peak A because we know that the conjugate base is gonna be predominate ing and most of the molecules are gonna be deep throat nated. 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, uh, ratios that we've drawn here indicate the approximate concentrations. So essentially, what we're saying here is, uh, these sizes are representing what we see and this column over here and so moving forward, we're gonna 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 re watch this video a couple times, and we'll be able to get some practice moving forward in our next couple of practice video. So I'll see you guys there
3
Problem
Problem
Consider the following pKa value for pyruvic acid. Which of the following species predominates at pH = 7.4?
