Acid-base titrations are an experimental approach to determine the concentration of an unknown acid or base.
Acid-base titrations are an experimental approach to determine the concentration of an unknown acid or base (0:56). Your episode starts by introducing the terms analyte and titrant (1:49) and looks at two examples: In the first example, we are titrating a strong acid with a strong base (2:03). We describe the shape of the titration curve (2:14) and define equivalence point (2:44), the point at which the number of moles of titrant and number of moles of analyte are the same. Using the equivalence point, we can calculate the molarity of the unknown analyte (3:03). The second example outlines the titration of a weak acid with a strong base (4:26) and describes why the equivalence point is at a pH higher than 7 (5:19). It also discusses the half-equivalence point (6:00), which can be used to determine the pKa of the weak acid.
Question: How many equivalence points will the titration curve of a diprotic acid show?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap titrations.
Lets Zoom Out:
Unit 8 - Acids and Bases
Topic 8.5 - Acid-Base Titrations
Big idea - Structure and Properties
In Episode 35 we encouraged you to take a look at our logo - if you STILL haven’t, now is the time! What happens if we add a base to an acid or vice-versa? And why would we even do that? Hey, hey, now IS actually the time to get salty! Let’s recap titrations.
Let’s zoom in:
Titrations are an experimental approach to determine the concentration of an unknown acid or base by adding, in a slow and controlled manner, either a base or acid of known concentration. To do it, we use the titrant, our solution of known concentration, that is added with the help of a buret, to the analyte, our acid/base with unknown concentration. It’s the solution we are analyzing, therefore, the “analyte”.
While slowly adding the titrant to the analyte, we are measuring the pH and can graph a titration curve, which shows the mL added vs the pH. Now how does that help us to determine the unknown concentration of the analyte? Let’s start with an example and recap some of the most important vocab and concepts.
In our example, the analyte is 10 mL of hydrochloric acid. It’s initial pH is 1. We are adding our titrant, sodium hydroxide, with a concentration of 0.1 M. This example is a reaction between a strong acid and a strong base, forming water and an ionic compound, in this case NaCl - salty, get it? Your titration curve will show that while you are slowly adding the sodium hydroxide, the pH is only rising very slowly. At this point, the concentration of the acid is much greater than the concentration of the added OH- ions. Then… all of a sudden, while you are adding one drop after another - and frankly, slowly getting impatient - the pH jumps from 4 to 10. WHAT happened?
You have reached the equivalence point, the point at which the number of moles of titrant and number of moles of analyte are the same! In the graph the equivalence point is the midway point of the steep rise. In our example, this point is at pH = 7 because we have a strong acid and a strong base. Knowing how much titrant you’ve added to reach the equivalence point, you can use stoichiometry to determine the concentration of the unknown analyte: The number of moles of titrant added is also the number of moles of analyte. Using the initial volume of the analyte, you can now determine the concentration. That sounds pretty straight forward, but what if I have the combination of weak and strong?
Let’s titrate a weak acid with a strong base, for example, hydrofluoric acid with sodium hydroxide. To understand this titration, we have to take a closer look at the reaction: hydrofluoric acid reacts fd;po;with the hydroxide ions to form the fluoride anion plus water. This reaction is an equilibrium reaction. Hydrofluoric acid is our weak acid, and the fluoride is the conjugate base. Let’s look at a few points of the titration curve: At the beginning, we only have our analyte, hydrofluoric acid. When adding the titrant, the pH will slowly rise as the HF molecules react with the hydroxide anions. Similar to the strong acid/base titration, all of a sudden, the curve will show a sharp increase in pH. The midpoint again is the equivalence point. BUT: You will notice that the equivalence point is not at pH 7, but higher. Why is that? At the equivalence point all HF molecules have reacted with hydroxide forming fluoride and water. Water itself at 25 degrees Celsius would have a pH of 7. But you also have fluoride, which reacts as a conjugate base with some of the water molecules, forming again HF and OH-, because we have an equilibrium reaction. Therefore, because of fluoride reacting as a base, the pH of the equivalence point is higher than 7.
Another point on this titration curve when titrating a weak acid with a strong base that is of interest is the half-equivalence point, which is when you’ve added half of the volume of titrant it takes to get to the equivalence point. At half-equivalence point, you have half of the hydrofluoric acid remaining and you have formed half of the conjugate base, the fluoride ion. Therefore, these two are equal. At this point, the pH equals the pKa! This shows that you can use the titration curve not only to determine the concentration of a weak acid, but also to determine the pKa of a weak acid!
The same principles apply, of course, when titrating a strong base with a strong acid or a weak base with a strong acid. Your titration curve will have the same shape, but start at the pH of the strong or weak base and the pH will decrease when adding the strong acid.
To recap:
Titrations are an experimental approach to determine the concentration of an unknown acid or base by adding either a base or acid of known concentration. A strong acid/base titration has an equivalence point at pH=7. At the equivalence point the number of moles of titrant and number of moles of analyte are the same. Titrating a weak acid or weak base with a strong acid/base will result in a pH higher/lower than 7, due to the reaction of the conjugate with water. At half-equivalence point, the concentration of weak species and conjugate are equal and therefore the pH=pKa.
Coming up next on the APsolute RecAP Chemistry Edition: buffers.
Today’s Question of the day is about polyprotic acids.
Question: How many equivalence points will the titration curve of a diprotic acid show?