Acid Dissociation Constant - Video Tutorials & Practice Problems

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1

concept

Acid Dissociation Constant (Ka)

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4m

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in this video, we're gonna talk about the acid dissociation constant. So now that we've reviewed pH, we know that the pH is a measure of the hydrogen ions in a solution. And so in this video, we're going to talk about those molecules that donate hydrogen ions to the solution. And those are acids. And every acid has an acid dissociation constant or a k a value. And the acid dissociation constant is literally just the equilibrium, constant foran, acids, dissociation. And we're already familiar with the equilibrium constant from our previous lesson videos and so down below in our image, you'll see that we have a typical reaction being shown where we have react and a being converted into two products. Product B and products seat. And so the equilibrium constant we know from our previous lesson videos is literally just a ratio. It's the ratio of the concentration of products at equilibrium over the concentration of reacting to equilibrium. And so we have to products, product be and products see. And then we have one reacting, which is reacting a which goes in the bottom. And so what you guys will notice here is that the acid dissociation. Constant is really nothing new. It's literally the equilibrium constant for an assets dissociation. And so here we're showing the acid dissociation reaction where we have a conjugate acid dissociating to form a conjugate base and a hydrogen ion. And so the acid dissociation constant or the K a value is literally the same thing as the equilibrium constant. It's the ratio of the concentration of products over the concentration of reactant, and so are two products are the H Plus and the conjugate base, which is represented as a minus. And then our one reactant is the conjugate acid, which goes in the bottom as H A. And so literally that is it, guys. Nothing really knew what this acid dissociation constant. It's essentially the same as the equilibrium, constant, just foreign acids association. And so what you guys should know is that the K A Value is really just a quantitative measure of the strength of an asset, and the K A value is also known as the ion constant. So sometimes you might hear professors say ion constant instead of acid association. And so the reason it's called the ion constant is because it expresses the tendency oven ion to disassociate from a molecule. And the ion that's dissociating from the molecule in our acid association is the hydrogen ion. Now, the greater the K a value is, the stronger the acid will be. So this is one of the more important things that you want to take moving forward. So we're able to determine which acid is the strongest asset and which asset is the weakest ass It just by comparing the K A values. And so some acids are known as Polly product acids and Polly Protic acids contain multiple acidic hydrogen atoms, and each acidic hydrogen atom can disassociate to form a hydrogen ion. And so what this means is that there's gonna be wand acid association constant for each acidic hydrogen. So over here in our image, we have an example of a poly protic acid, and that example is phosphoric acid, whose chemical formula is H three p 04 And actually, all three of these hydrogen in this molecule are acidic hydrogen. And because there are three acidic hydrogen, that means that there are three K A values for this one acid. And so what you'll see here is that The first K value is for the first acidic hydrogen, the second K values for the second acidic hydrogen, and the third K value is for the third acidic hydrogen. And so just by comparing these K A values down below, which one do you guys think is the strongest acidic hydrogen? That's right. So recall that the greater the K A value is, the stronger the acid is. And so the hydrogen that's the strongest acid is gonna be the one with the greatest K A. And that is the first K over here. And so the first acidic hydrogen is the strongest asset of the three. And so we'll be able to apply these concepts that we've reviewed here in our future videos, so I'll see you guys in our next video.

2

example

Acid Dissociation Constant (Ka)

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5m

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Alright guys. So here's an example problem. And it's asking us to calculate the K or the acid dissociation constant of uric acid whose chemical formula is shown here. If the concentration of this molecule here it equilibrium is 4.7 times 10 to the negative third Mueller in the concentration of this molecule over here in equilibrium is 7 to 7 times 10 to the negative fourth Mueller. And so what we'll need to realize is this molecule over here on the left is the conjugate acid of uric acid. And we know because it has one additional hydrogen, it has four hydrogen and the molecule on the right is the conjugate base of uric acid because it has one less hydrogen and one less charge, a negative charge. And so because we're being asked to calculate the K And we know from our previous lesson videos that the K is literally just the equilibrium, constant oven acids dissociation. I provided the acid dissociation reaction over here where we have the conjugate acid represented by H A associating into a minus or the conjugate base and a hydrogen ion. And so because the K is literally just the equilibrium constant. We know it's the ratio of the concentration of products at equilibrium over the concentration of reactant at equilibrium. And so because we're working with uric acid here, I've also provided uric acids dissociation, where we know that the conjugate acid eyes going to disassociate into the conjugate base molecule as well as the hydrogen ion. So we have the same ratio presented over here. And so essentially, what we'll need to realize is we have the values for all of these, um, concentration. So we have the value of the concentration of conjugate base, which is 7.27 times 10 to the negative fourth and we have the value for the conjugate acid concentration, which is 4.7 times 10 to the negative third. And really, the Onley concentration that we're missing here is the hydrogen ion concentration and the way that we can determine the hydrogen ion concentration is by using an ice table on ice chart, which I know you guys remember from your previous chemistry courses. So we're going to do a quick little refresher here, and so essentially, a nice table is where you have the initial concentration the change in the concentration over time and then the equilibrium concentrations. And so initially, what we can imagine is that all of the uric acid that was present in the solution was present as the conjugate base. And so what that means is we were gonna have some initial concentration acts, some unknown initial concentration of uric acid. And at the very beginning of the reaction, we would have zero of our products, so we would have zero of the hydrogen ion and zero of the conjugate base. And so we know that over time, if we have all of this uric acid, it's going to essentially disassociate. It's gonna react and associate into conjugate base. And we're told that at equilibrium we know it's at equilibrium because of the E. Q. Here we have a concentration of 7.27 times 10 to the negative fourth of our conjugate base. And what that means is the change in concentration here for the conjugate base must have been an increase. A positive 7.27 times 10 to the negative fourth Moeller increase. And so what that means is that the conjugate acid here must have decreased by that same amount. So a negative 7.27 times 10 to the negative fourth Moeller. And because the conjugate base is at a one toe, one Mueller ratio with the hydrogen ion, Uh, because they both have coefficients of one, that means that the hydrogen ion change in concentration is going to be the same as the conjugate base here. So it's gonna be a positive 7.27 times 10 to the negative fourth Mueller. And so what that means is, uh, that the hydrogen ion concentration at equilibrium is also going to be 7.27 times 10 to the negative fourth Moeller. And of course, we're told up above that at equilibrium for the conjugate acid, we have a concentration of 4.7 times to the negative third Mueller. And so recall that with the as the dissociation constant, we're using the concentrations at equilibrium. So we want to plug in these concentrations down below, and we've already plugged in the conjugate acid and the conjugate base concentration. So now all we need to do is plug in this number here for this, uh, hydrogen ion concentration and so we could go ahead and erase this and type in 7.27 times, 10 to the negative fourth year. And so now we have all of our numbers. We could go ahead and plug in all of these numbers into our calculator. And essentially, what we'll get is an answer of about 1.3 times 10 to the negative fourth. And this here is our answer. So this is the K or the acid dissociation constant, which matches with answer option D. So we could go ahead and indicate that D here is the correct answer. And so that concludes this example problem, and we'll be able to get some practice in our next lesson video, so I'll see you guys there.

3

Problem

Problem

Calculate K_{a} of proprionic acid (CH _{3}CH_{2}CO_{2}H) if [CH_{3}CH_{2}CO_{2}H]_{eq }= 0.2 M & [CH_{3}CH_{2}CO_{2}^{-}]_{eq }= 1.62 x 10^{-3} M.

A

K_{a} = 1.3 x 10^{-5}

B

K_{a} = 7.8 x 10^{-10}

C

K_{a} = 3.9 x 10^{-12}

D

K_{a} = 5.1 x 10^{-4}

4

concept

Acid Dissociation Constant (Ka)

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3m

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So in our previous lesson video, we talked about the acid dissociation constant, or the K A, which we know is a quantitative measure of the strength of an acid. And the greater the K A value is, the stronger the acid will be. But the thing about the K A values is that sometimes they are inconveniently large or small numbers that need to be expressed with scientific notation because of how large and small they could be. And that can make calculations unnecessarily mawr complicated. But the good thing is, is that the K A values can be expressed on a longer rhythmic scale with PK A values. And so the PK A values are also a quantitative measure of the strength of an acid, just like the K A. Value is, however, the relationship is different. And so again, recall with the K A values, the greater the K a, the stronger the acid. But with the PK, it's different. And so the greater the PKK, the weaker the acid is, and so it's actually an inverse relationship. And so, in our example, down below, we have an equation for the P K A. And just like from our previous lesson of pH. We know that the P here is really just representing the negative log. And so the P K is just the negative log of the K A value. And that's exactly what we have over here. The negative log of the K A. Now the rules of logarithms say that the negative log is equal to the positive log of the reciprocal. So we have both equations shown here, just in case your professors leaning one way or the other. So now we'll be able to get a little bit of practice utilizing this equation as we take a look at our chart over here on the right, which has an example of a weak acid of acetic acid, and this yellow column and an example of a strong acid HDL or hydrochloric acid in the Blue column, and were given the K A values for both of these acids and noticed that the K a value of the weak acid is an incredibly incredibly small. And we know that it's small because it's expressed in scientific notation with a negative exponents, and the strong acid, on the other hand over here, has an incredibly large K a value. And again we know it's large because it's expressed in scientific notation with a positive exponent. And so these numbers that are inconveniently large and small again can be unnecessarily over complicating calculations. But we can pretty easily convert them into PK a values. And so we can use this equation over here for the PKK. And so, for the PKK, we know that we can take just the negative log of the K A values. So for the weak acid, it will be the negative log of 1.76 times 10 to the negative fifth, which is equal to 4.76 So that's the PK for the weak acid. And then, if we do the same for the strong acid over here, essentially we'll take the negative log of the K A, which is 1.3 times 10 to the sixth, and that is equal to negative 6.3. And this is the PKK for the strong acid. And so what you'll notice is that the PKK for the weak ass is actually larger, then the PKK for the strong acid. And so that's because again it's an inverse relationship. The greater the PK, the weaker the acid is, and so moving forward, we'll be able to get a little bit of practice utilizing these concepts, so I'll see you guys in those videos.

5

Problem

Problem

Which of the following is the strongest acid listed?