Principal Species - Video Tutorials & Practice Problems

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Depending on the pH of the solution different forms of protic product can exist.

Principal Species

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Principal Species

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So we've learned in the past that are acid dissociation constant. Otherwise known as K. A. Is a way of gauging the strength of an acid. We know that if R. K. Value is greater than one then we're dealing with a strong acid. And if our K. is less than one then we're dealing with a weak acid. Now in addition to this we can say that the acid constant K. A. Tells us numerical value that an acidic hydrogen can be removed. So basically if we can convert our K. Two P. K. We can compare it to the ph of our solution to determine which one is the principal species within our solution. Is it the acidic form or the basic form that predominates. Now here if we're looking at a model product acid, if we have the ph being less than R. P. K. A. Then we're gonna say that the acid form is greater than the basic form. And then if we look at it the other way, if our ph is greater than R. P. K. A. Then our asset form is less than our basic form. The way you can think of it is we have these two forms H. A. And a minus. And in between them is this line here at this line we can say that this is our P. K. A. If your ph happens to be less than this P. K. A. Then we'll exist in our acidic form here. But if all of a sudden we crank up the ph where it surpasses P. K. A. Then we'll be on this side of the fence. So exist in our basic form. Now what would your um what would be the principal species if your ph equaled your P. K. Well if your ph equaled your P. K. You'd be exactly on this dividing line here and on this dividing line, both forms exist together. So if your ph equal equal your P. K. A. That would mean that you have 50% of this acid form and 50% of the basic form. And that is true in P. H. Equals P. K. A. Now this relationship of PH and PK can also be expanded outwards to die product as well as polyp product acids. So we'll take a look at those situations as we investigate each of the example problems left below. So click on the next video and see how we approach. Example one.

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example

Principal Species

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So in this question here it says, what is the predominant form of the dia protic acid? Matthias nine at a ph equal to 4.18. Alright, so we're dealing with the dia protic acid. Remember when it comes to die protic species, there are three major forms We have are fully propagated form. So we can say that is H. Two A. So you can think of this as the acidic form. Then we have our intermediate form H H A minus. And finally we have our basic form. After all the acidic hydrogen have been removed. So that is a two minus. And remember when we're talking about basically borders and things. So, in between our acidic form and the intermediate form exists one border here. So we're talking about removing the first acidic hydrogen from H. Two A. To create a minus. Since we're talking about the first acidic hydrogen. That means we're dealing with K on K one here, we're gonna take the negative log of that to find P. K. One. So P. K. One equals the negative log of K one. So taking the negative log of that gives me 2.18. So here on this border, P. K. One equals 2.18. So, remember exactly on that border, we'd have 50 Of the acid form and 50% of the of the intermediate form in this case. Now, the next border that separates my intermediate form from the basic form would be here. And in that case we'd be dealing with P. K. A. Two Because we're removing the second acidic hydrogen to create our basic form, P K. A two equals negative log of K two. So that's negative log of 8.3 times 10 to the -10. So that gives me 9.08. So here we have 50% of the intermediate form And 50% of the basic form. Now here we're dealing with the ph equal to 4.18, which means that we've passed beyond this border here and we're not quite at this border here. We're somewhere in the middle. And what form exists predominantly in that space? The intermediate form does. So here the intermediate form will be the predominant form because it is past the first border. So it's blown past the acidic form, but it hasn't quite reached the second border. So it's not having um any equal equal amount with the basic form. So in this case, our intermediate form of Matthias nine would be the predominant species. Now that you've seen this example, see if you can do example to once you do come back and see how I approach that same question

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example

Principal Species

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So although we didn't cover one dealing with a try protic acid, we can apply the same methods that we use in the previous example here, it says what is the predominant form of histamine at a ph equal to eight. Here, we know it's a try protic acid because it has three P. K. Values.

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example

Principal Species

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So if we take a look at this example three, we said what is the second most predominant form in the previous question. So we know that the predominant form is the intermediate to form This ph of eight which is here is within this region. Next to that region is the first intermediate form. And our basic form, we're trying to determine which one of these two would be the next highest um concentration at a ph of 8.0. Now, I gave the hint of looking up ice electric point here. If we calculate the icy electric ph that will help us navigate, which will be the second most dominant form. So here P. H equals half P. K. A. Plus P. K. A. Now which pks are we dealing with? Well 8.0 is in between P. K. Two And P. K. three. So those would be the pks that we're using. So that equals half of 5.97 plus 9.28. We're gonna figure out the ph here. When we do we get 7.625. As my ph my Isil Elektrik ph Now, what do we say in terms of this value? Well, we're gonna say here, If we are above a ph of 7.625. That means we are more basic, right? Because the higher ph gets, the more basic your solution becomes and the more basic your solution becomes, the easier it is to d protein eight or take off an H. Plus. So at a ph above 7.625. It becomes too basic. So the second dominant form would be a three minus. So a three minus basic form would be second most predominant. And then if we sat below A ph of 7.625 that would mean that our ph is lower. So it's more acidic. And remember in a more acidic environment what do we do? We add an H. Plus to something we protein ate it. So in that form H two a minus which is the intermediate one would be second most predominant. So this whole ice electric ph formula is just to help us establish a baseline and all we do now is we compare uh this big new baseline to the current ph environment, the current ph environment is 8.0. Like we set an example to. So the current ph um ph value is higher than our baseline of 7.625. Therefore it's the basic form that would be the second most predominant form within the solution. So that's how we are able to determine who is the second most popular or dominant form within a solution. So refer to the borders um examples for example, wanted to to find the most dominant and then refer to an ice electric ph to determine which one is second. Once you do that you'll be able to get your answers