Buffers - Online Tutor, Practice Problems & Exam Prep

Buffer solutions consist of a weak acid and its conjugate base, or a weak base and its conjugate acid, allowing them to resist pH changes. A good buffer maintains a 10:1 to 1:10 ratio of weak acid to conjugate base. Buffer capacity increases with concentration. Mixing a strong acid with a weak base requires the weak base to be in greater amount, while mixing a weak acid with a strong base also necessitates the weak acid to be in higher concentration. Understanding these principles is crucial for effective buffer preparation and application.

Buffers Concept 1

Hey guys, in this new video, we're going to take our first looks at what exactly is a buffer and how exactly they work. So, we're going to say here solutions that contain a weak acid and its conjugate base. Remember, when I say weak acid, we can think of HF. Conjugate base means it has one less H⁺. So take off the H⁺, you have F^-. But professors don't like to give you just F^-, they'll combine that F^- with a metal. So we could say our conjugate base here is NaF. So solutions which contain a weak acid like HF and a conjugate base with one less H like NaF are called buffer solutions because they resist drastic changes in pH.

Now, how do they accomplish this? Well, they do this by keeping your H⁺ concentration and your OH^- concentration constant. It keeps them basically the same. The way this works is we're going to say adding a small amount of strong base and the pH will increase because remember, if you add base, you're going to go above 7, so you keep going up and up. But it's not going to increase by much because the weak acid will neutralize the strong base that you add. Now adding a small amount of strong acid and the pH will decrease. Again, it's not going to decrease by much because why? The conjugate base will neutralize what you add. So that's how a buffer works. Any acid or base that I add, they get reacted with either the weak acid or the conjugate base. Remember, acids and bases are natural enemies of each other. If I add base, my weak acid steps in to try to take it out. If I add acid, my conjugate base steps up and tries to take it out. That's how a buffer will work.

And remember, the only things that can destroy a buffer are by adding too much strong base or too much strong acid. Adding water to a buffer does nothing to it. Why? Because adding water changes the concentration of my weak acid but also changes the concentration of my weak base proportionally. So proportionally, the ratio would stay the same, so my buffer would be unaffected by the addition of water. So remember, adding a strong acid or base is the only way to really destroy a buffer. Adding water does nothing to it.

Now, knowing this, I want you guys to try to answer this first practice question. We're going to say, which one of the following combinations does not create a buffer? Remember, we just said above, a buffer is a weak acid and its conjugate base. So this is going to require you guys to remember the rules that we talked about in identifying compounds as either being acidic or basic, strong or weak. Once you can do that, you'll be able to guide your way to the correct answer for this one. Come back, click on the explanation button, and see the choice that I make and see if it matches up with yours.

Which one of the following combinations does not create a buffer?

• A. HC₂H₃O₂ and KC₂H₃O₂
• B. H₂SO₃ and NaHSO₃
• C. H₃PO₄ and NaH₂PO₄
• D. HNO₃ and KNO₃
• E. NH₄Cl and NH₃

Which of the following combinations can result in the formation of a buffer?

• A. HF and HI
• B. HC₂H₃O₂ and NH₃
• C. CH₃CH₂NH₂ and CH₃CH₂NH₃⁺
• D. NaCl and NaOH

Buffers Concept 2

Hey guys. In this new video, we're going to take a look at what are the different ways to create a buffer. Alright. We know what a buffer is. A buffer is made up of a weak acid and conjugate base or a conjugate acid and a weak base. So that's what a buffer is. But there are 3 ways to make a buffer and it's going to be important that you guys recognize these three ways. Now, we're going to say the first way is obvious. A buffer is made up of a weak acid and its conjugate base. So one way to make a buffer is just mixing those two things together. So we're going to say mixing a weak acid and its conjugate base. For example, what we need to realize here is that we're going to say in this case, a good buffer, an ideal buffer. Ideal buffer means that this is the best type of buffer. An ideal buffer is when our weak acid equals our conjugate base. They're the same amount. Let's say we have 0.1 molar HF and 0.1 molar NaF. And we should realize that the numbers could be different for them, but they work best when they're the same. We're going to say this has to do with our buffer range. What you should realize here, a good buffer has to fall within the range of 10 to 1 or 1 to 10. Okay. So what do I mean by that? I mean this. So 10:1, that means our weak acid at most could only be 10 times more than my conjugate base. If it falls outside that range, it'll be a bad buffer. Okay, so a good buffer is between a 10:one ratio or 1:10 ratio. In the 1:10, now the weak acid is 1 and the conjugate base is 10 times that. Okay? So again, this is the buffer range. A good buffer falls within this range. If it falls outside that range, it'll still be a buffer. It'll just be a very bad buffer. And remember, an ideal buffer is the best buffer. That's when both are the same. Now, connected to this also is another topic which is called our buffer capacity. Now we're going to say buffer capacity, all it means is, the more concentrated my weak acid and conjugate base, the better my buffer. And this makes sense because remember, what is a buffer doing? It's trying to neutralize the strong acid and the strong base that you add to them. And they can't do their job effectively if there's a little bit of them. So to fight such strong acids and strong bases, you need a lot of weak acid and a lot of conjugate base. The more the better, the more they can better defend themselves. So here we're going to say, this is good because they're both the same number, but this is better because it's more concentrated. This has a better buffer capacity. So buffer capacity, the more concentrated the better. Now, if we go back to this ideal buffer, we're going to say that's when they both equal each other. We're going to say that this is found at our half equivalence point. When we reach the half equivalence point, that's when we have an ideal buffer. We'll talk more in detail on that when we get to titration graphs. When we get to the titration graphs of buffers, we'll take a close look and see what do I mean by the half equivalence point and why is this an ideal buffer. So just for right now, write a little note on that. An ideal buffer is when they're both equal. This happens at the half equivalence point. So remember the difference between buffer capacity and buffer range. The best buffer is when they're both equal in amount. The buffer range says they could be a 10 to 1 ratio. Once it's outside that 10 to 1 ratio, the buffer is still a buffer but it's going to be a bad buffer. It'll get destroyed very quickly by a strong acid or base that we add.

Buffers Concept 3

Now, the second way to make a buffer is to mix some strong acid with some weak base. But here's the thing about this type. We're going to say here the weak base has to be higher in amount, must be higher in amount. If the weak base is equal to the strong acid, it'll destroy the buffer. If the strong acid is greater than my weak base, it'll destroy the buffer. Okay, so the weak base has to always be higher than the strong acid. Otherwise, the buffer will be destroyed. So if we take a look at an example, we can say here we have 1 mole of HF and 1.5 moles of NH₃. NH₃ is our weak base and there's more of it. Our unit here could be moles or molarity. In this case, this would be a buffer. If we have 1 mole of HF and 1 mole of NH₃, no longer a buffer because the strong species is equal to my weak species. If I have 1.5 moles of HCl and still one mole of NH₃, this would also no longer be a buffer. Your strong species cannot be equal to or greater than your weak species. If it is, your buffer gets destroyed. So remember that the weak species has to be higher in amount than the strong species.

Buffers Concept 4

And then finally, the last type, we just said strong acid and weak base, so let's reverse it, weak acid and strong base. Example here, we could have 1. Let's do a different number 2.50 molar HF now, that's a weak acid, and 1.0 Molar NaOH. Now, our weak acid is definitely larger in amount. So again here, we need the weak species higher in amount. So this would be a buffer. Got to remember that.

So if we go back, the first way is to mix a weak acid and conjugate base. Here, they could be equal to or different from each other but the good buffers are within the range of 10¹. In the next two, the weak species has to be higher than the strong species. Otherwise, it won't be a buffer. You have to remember all three cases because you'll be tested on them. And if you don't know which is which, you won't be able to get the correct answer. So just remember, with a weak base, they can be equal, they can be different. As long as it's within the range of 10¹, it's a good buffer. If it's outside that range, it's a bad buffer.

Case 23, the weak species have to be higher than the strong species. The strong species cannot be equal to or greater. If it is, the buffer gets destroyed. Remember these principles, and they'll help guide you to see if you have a buffer or not.

Buffers Example 1

Hey, guys. In this new video, we're going to put to practice some of the principles we learned in creating a buffer. So for example, when it says, which of the following combinations can result in the formation of a buffer? So what we should realize here is in every example, we have 0.01 moles of hypochlorous acid. Now, hypochlorous acid, it says acid so we know it's some type of acid. It has oxygen so it's an oxyacid. And remember, if we do the math, 1 oxygen minus 1 hydrogen gives us 0 left over. So hypochlorous acid is a weak acid. So every single one of these is a weak acid. And because of that, you have to remember, what two ways can we make a buffer if we have a weak acid? So remember, the two ways is if we have a weak acid and its conjugate base. The second way is if we have a weak acid and a strong base. But in this second case, the weak species has to be higher in amount. So let's take a look at what's mixing with. Here we have NaOH which is a strong base. Here we have HCl which is a strong binary acid. Here, NH₃ is a neutral amine, so it's a weak base. And NaOH again is a strong base. So we're looking for the weak acid to be paired up with the conjugate base or the strong base. So automatically, these 2 are out because we can't pair up a weak acid with a strong acid or a weak acid with a weak base. It has to be a conjugate base or strong base. Now, in the first one, we actually have more moles of the strong species, so this would not be a buffer. This would actually destroy the buffer. But for the last one, we have more of the weak species than the strong species. So it would be the last one that makes a buffer. So again, remember the 3 principles that we learned in creating a buffer. Apply them to this type of question.

Now for practice 1, I want you guys to figure out which one of these combinations will result in a buffer. Here, remember the word "of" means multiply. So if you multiply these two numbers together, you can help to find moles because remember, moles equals liters times molarity. So divide each of these by 1000, multiply it by the molarity to have moles. You could also keep it in milliliters and multiply times the molarity, but then that would give you millimoles. So I don't know if you guys want to deal with millimoles. It's basically the same thing but you don't have to do it that way. So divide them by 1000, the mL to get liters, multiply times the molarity to give you moles. Remember the 3 combinations that can result in the formation of a buffer. So check for each one of these. Look to see, do you have a weak acid and conjugate base? Do you have a weak acid and strong base? Or do you have a strong acid and weak base? Look for these combinations. If you don't see them, don't bother doing the math. So I would recommend looking at each compound first and seeing if they match up with any of these combinations. If they do, look more deeply. Look to see, does it help to create a buffer? If it doesn't match one of these three combinations, don't even bother looking at it. Don't do the math at all. And remember, one key thing I'll give you guys a hint, You have to make sure that you have the correct concentration of your strong bases. What did we say about strong bases? The concentration I give you is dependent on how many ions you have of which particular 4 ions. Doing that will help to get the correct answer. Once you're done with this, come back and click on the explanation button and watch me explain how to best approach this problem. Good luck, guys.

 Which of the following combinations can result in the formation of a buffer?

• A. 50 mL of 0.10 M HF with 50 mL of 0.10 M NaOH.
• B. 50 mL of 0.10 M HNO₂ with 25mL of 0.10 M Ca(OH)₂
• C. 50 mL of 0.10 M CH₃CO₂H with 60 mL of 0.10 M NaOH
• D. 50 mL of 0.10 M HF with 30 mL of 0.10 M NaOH.

A buffer solution is comprised of 50.0 mL of a 0.100 M HC₂H₃O₂ and 60.0 mL of a 0.100 M NaC₂H₃O₂. Which of the following actions would completely destroy the buffer?

• A. Adding 0.003 mol HC₂H₃O₂
• B. Adding 0.007 mol Ca(C₂H₃O₂)₂
• C. Adding 0.005 mol NaOH
• D. Adding 0.004 mol HCl
• E. Adding 0.001 mol HCl

Here’s what students ask on this topic:

What is a buffer solution and how does it work?

A buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. It works by resisting drastic changes in pH when small amounts of strong acid or base are added. This is achieved by the weak acid neutralizing added bases and the conjugate base neutralizing added acids. For example, in a buffer containing HF (weak acid) and NaF (conjugate base), HF will neutralize any added OH^- ions, while F^- will neutralize any added H⁺ ions. This keeps the pH relatively constant.

How do you create a buffer solution?

There are three main ways to create a buffer solution: 1) Mix a weak acid with its conjugate base, such as HF and NaF. 2) Mix a weak base with a strong acid, ensuring the weak base is in excess, like NH₃ and HCl. 3) Mix a weak acid with a strong base, ensuring the weak acid is in excess, such as HF and NaOH. The key is to maintain the appropriate ratio of weak acid to conjugate base (or weak base to conjugate acid) to ensure effective buffering.

What is buffer capacity and how is it determined?

Buffer capacity refers to the ability of a buffer solution to resist changes in pH upon the addition of an acid or base. It is determined by the concentration of the weak acid and its conjugate base (or weak base and its conjugate acid). The higher the concentration of these components, the greater the buffer capacity. This is because a more concentrated buffer can neutralize more added acid or base before the pH changes significantly.

What is the ideal ratio of weak acid to conjugate base in a buffer solution?

The ideal ratio of weak acid to conjugate base in a buffer solution is 1:1. This occurs at the half-equivalence point during a titration, where the concentrations of the weak acid and its conjugate base are equal. However, a good buffer can also function within a range of 10:1 to 1:10. Outside this range, the buffer's effectiveness decreases, and it may not adequately resist pH changes.

Why does adding water not affect a buffer solution?

Adding water to a buffer solution does not affect its pH because it dilutes both the weak acid and its conjugate base (or weak base and its conjugate acid) proportionally. This means the ratio of the concentrations of the weak acid to its conjugate base remains constant, preserving the buffer's ability to resist pH changes. The buffer's capacity may decrease due to dilution, but its pH will remain stable.