We've learned in the past that our acid dissociation constant, otherwise known as Ka, is a way of gauging the strength of an acid. We know that if our Ka value is greater than 1, then we're dealing with a strong acid. And if our Ka is less than 1, then we're dealing with a weak acid. Now, in addition to this, we can say that the acid constant Ka tells us the numerical value that an acidic hydrogen can be removed. Basically, if we can convert our Ka to pKa, 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 monoprotic acid, if we have the pH being less than our pKa, then we're gonna say that the acid form is greater than the basic form. And then if we look at it in the other way, if our pH is greater than our pKa, then our acid form is less than our basic form. The way you can think of it is we have these two forms, and A minus. And in between them is this line here. At this line, we can say that this is our pKa. If your pH happens to be less than this pKa, then we'll exist in our acidic form here. But if all of a sudden, we crank up the pH where it surpasses pKa, then we'll be on this side of the fence, so exist in our basic form.

What would your, what will be the principal species if your pH equaled your pKa? Well, if your pH equaled your pKa, you'd be exactly on this dividing line here. On this dividing line, both forms exist together. If your pH equaled your pKa, that would mean that you have 50% of this acid form and 50% of the basic form. That is true when pH equals pKa. This relationship of pH and pKa can also be expanded outwards to diprotic as well as polyprotic acids. We'll take a look at those situations as we investigate each of the example problems left below. Click on the next video and see how we approach example 1.