Determining Predominate Species: Study with Video Lessons, Practice Problems & Examples

To determine the predominant species of a molecule, compare the solution pH to the acid's pK_a. When pH equals pK_a, concentrations of conjugate acid and base are equal. If pH is less than pK_a, the conjugate acid predominates; if greater, the conjugate base does. This relationship is encapsulated in the Henderson–Hasselbalch equation: pH=pK_a+log(C_B/C_A). Understanding these concepts is crucial for analyzing acid-base equilibria.

Determining Predominate Species

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

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 pK_a value for pyruvic acid. Which of the following species predominates at pH = 7.4?

• A. Conjugate base (CB)
• B. Conjugate acid (CA)
• C. Neither predominates ([CA] = [CB])
• D. Not enough info to tell.

Here’s what students ask on this topic:

What is the Henderson-Hasselbalch equation and how is it used to determine the predominant species of a molecule?

The Henderson-Hasselbalch equation is used to relate the pH of a solution to the pKa of an acid and the ratio of the concentrations of its conjugate base and conjugate acid. The equation is:

$\mathrm{pH}=\mathrm{pKa}+log\left(\frac{C{B}_{}}{C{A}_{}}\right)$

To determine the predominant species, compare the solution pH to the acid's pKa. If pH = pKa, the concentrations of conjugate acid and base are equal. If pH < pKa, the conjugate acid predominates. If pH > pKa, the conjugate base predominates. This helps in understanding the acid-base equilibria in the solution.

How do you determine the predominant species when the pH is equal to the pKa?

When the pH of a solution is equal to the pKa of the acid, the concentrations of the conjugate acid and conjugate base are equal. This means that neither species predominates, and the solution contains 50% conjugate acid and 50% conjugate base. This equilibrium state is crucial for buffer solutions, which resist changes in pH upon the addition of small amounts of acid or base.

What happens to the predominant species when the pH is less than the pKa?

When the pH of a solution is less than the pKa of the acid, the conjugate acid predominates. This is because the lower pH indicates a higher concentration of hydrogen ions (H⁺), which shifts the equilibrium towards the protonated form of the molecule. Therefore, the majority of the molecules will be in the conjugate acid form, which is protonated.

What is the relationship between pH, pKa, and the concentrations of conjugate acid and base?

The relationship between pH, pKa, and the concentrations of conjugate acid and base is described by the Henderson-Hasselbalch equation:

$\mathrm{pH}=\mathrm{pKa}+log\left(\frac{C{B}_{}}{C{A}_{}}\right)$

When pH = pKa, the concentrations of conjugate acid and base are equal. If pH < pKa, the conjugate acid predominates, and if pH > pKa, the conjugate base predominates. This equation helps predict the predominant species in a solution based on its pH and the acid's pKa.

Why is it important to understand the concept of predominant species in biochemistry?

Understanding the concept of predominant species is crucial in biochemistry because it helps predict the behavior of molecules in different pH environments. This knowledge is essential for enzyme activity, drug design, and metabolic pathways, as the ionization state of molecules can affect their function, solubility, and interaction with other molecules. By knowing the predominant species, biochemists can better understand and manipulate biochemical processes.