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 Hasselbalch equation from your previous chemistry courses, and we know that the Henderson Hasselbalch equation reveals the ratio of the concentration of the conjugate base to the concentration of the 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 pKas of acids to determine the predominant species of those acidic molecules. So I'll see you guys in that lesson video.
Determining Predominate Species - Video Tutorials & Practice Problems
Determining Predominate Species
Video transcript
Determining Predominate Species
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. Recall that the conjugate bases are deprotonated molecules because they have one less hydrogen than the conjugate acid. The conjugate acids, of course, are going to be protonated molecules, and that's because they have one more hydrogen than the conjugate base. In the example below, we're going to compare the pH and the pKa to fill in all of the blanks in our chart below. When we say compare the pH to the pKa, we mean to 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. 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. By always associating the pKa with the concentration of conjugate acid, I'm able to quickly determine the predominant species. Let me show you what I mean.
When the pH is equal to the pKa, I know that the concentration of conjugate base CBCA is going to be equal to the concentration of conjugate acid. Pretty easy, right? When the pH is less than the pKa, 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. Essentially, it's going to have the same relationship as the pH over pKa. And so, when the pH is greater than the pKa, again, because I'm associating pKa with the concentration of conjugate acid, I know that the concentration of conjugate base will be greater than the concentration of conjugate acid. That's pretty easy, right? Associate the pKa with the conjugate acid, and you'll be able to quickly determine the predominant species. The predominant species will be the one in the highest abundance.
Notice 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 neither is in the 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 going to be in a protonated conjugate acid form. When the pH is less than the pKa, the conjugate acid is in higher concentration, which means the conjugate acid is predominating. We know that the majority of the molecules are going to be protonated, since our conjugate acid is always protonated. However, when the pH is greater than the pKa, the conjugate base is in higher concentration so the conjugate base predominates. We know that the conjugate base is a deprotonated form, so that means that the majority of the molecules are going to be deprotonated.
In our final column over here on the far right, notice we have the Henderson-Hasselbalch equation, which we know is pH=pKa+logCBCA. You'll notice here that we have the same exact equation for all three of these different scenarios, and the only thing that's going to be different is this ratio of the concentration of conjugate base to the concentration of conjugate acid. When the pH is equal to the pKa, we know that the concentration of conjugate base, which I'll abbreviate with CB, is going to be relatively equal to the ratios of the conjugate acid, abbreviated CA. 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 going to have a large concentration of conjugate acid, and on the other hand, we're going to have a much smaller concentration of conjugate base. With this last scenario, when the pH is greater than the pKa, because we know that the conjugate base is going to be predominating and most of the molecules are going to be deprotonated, indicating that we have a large concentration of conjugate base, and a smaller concentration of conjugate acid. These sizes represent what we see in this column over here. Moving forward, we'll utilize a lot of these concepts, and it's important for you to take some time to study this chart and to commit some of it to your understanding. Rewatch this video a couple of times, and we'll get some practice moving forward in our next couple of practice videos. I'll see you guys there.
Consider the following pKa value for pyruvic acid. Which of the following species predominates at pH = 7.4?