Intro to Buffers - Video Tutorials & Practice Problems
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1
concept
Acid-Base Buffers
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3m
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Acid based buffers are solutions that resist drastic changes in pH by neutralizing additional acid or base that might be added to it. Here we're going to say that a buffer itself contains both acid and base, and because of that it neutralizes any additional hydroxide ion or hydronium ions that are added respectively. If we take a look here in this far left image here, this beaker represents our buffer solution. Our buffer is composed of hydrofluoric acid and its conjugate base in the form of sodium fluoride. Here, the buffer has a pH around 8. What I can do to it is I start adding acid or base. If I start adding hydrobromic acid, hydrobromic acid represents a strong acid. Remember, in terms of neutralization reactions, what is the chemical opposite of this hydrobromic acid? The opposite would be the sodium fluoride, the conjugate base. So as we add hydrobromic acid to the solution the fluoride ion, which is basic, would interact with the hydronium ion of hydrofluoric acid, They would bind together and as a result create hydrofluoric acid as well within the solution. We're creating acid. And then we're going to say here that the bromide ion and the sodium ion would still be present as well, but there are spectator ions because they're neutral ions. Alright, so as I add strong acid my buffer is slowly neutralizing it, my conjugate base is slowly neutralizing it. Neutralizing it does create some acid. This causes my pH to drop, but because it's a buffer the pH is not going to drop by much. So here this would be 7.8 as an example. So drop, but but not by much because we're assuming we're not adding too much of the strong acid. But let's say we added strong base, what would happen there? Well, if I add strong base its chemical opposite is an acid, so the hydrofluoric acid of the buffer would interact with that strong base being added. Here we'd say that the hydroxide ion of the strong base and the hydronium ion of the hydrochloric acid would interact, they would neutralize each other and we create water. Then we'd say that we have sodium ions that are free floating now, and we'd have more fluorine ion freed up, so they'd be floating around. Fluorine ion is a basic ion, so it will cause my solution to become a little bit more basic. As a result, our pH maybe goes up 8.2. Again, I'm just showing you that the addition of a strong acid or strong base does change the pH, but it's not going to change it by much because the whole point of a buffer is to resist large changes in your pH as long as not a lot of strong acid or strong base is added.
2
concept
Buffer Creation
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3m
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At this point we know that a buffer contains both an acid and a base, but let's be a little bit more specific. Here we're going to say when it comes to buffer creation there are 3 ways to create a buffer. Now, the first way to make a buffer is to have either a weak acid or base with its conjugate. So for example here we have 0.40, molar of ammonia and 0.40 molar of Ammonium ion. We can look at this two different ways. We could say that n h three represents a weak base, and the ammonium ion represents its conjugate acid, or we could say that ammonium ion represents a weak acid because it's a positive amine, and ammonia represents its conjugate base. Whichever way you look at it you're technically correct. Right? So the first way to make a buffer is to have a weak acid or base with its conjugate. Now an ideal buffer, which is the best type of buffer, this is the one that makes the most resistance to changes in pH. This happens when your weak acid or base is equal in concentration to its conjugate. So here we have 0.40 of both of these compounds and ions, this would be an ideal buffer. Now the next two ways. The second way to make a buffer is to have a strong acid with a weak base. Here we'd say that the weak base needs to be greater or higher in concentration. Okay. So we have something strong versus something weak. The weak species needs to be greater in amount. So for example here we have 0.20 molar of hydrochloric acid, which is a strong acid, and then here we have methylamine, it's a neutral amine so it represents a weak base. For us to have a successful buffer the weak base must be higher in amount than the strong acid, so here let me put point 30 molar. Here the weak base is greater in amount so this could create a buffer. Now, the final way to make a buffer is to have the opposite. Now my base is strong and my acid is weak, but again the weak species, in this case the weak acid, needs to be higher in concentration or higher in amount. Here we have 1.3 molar of potassium amide, which is a strong base, And here we have, sulfuric acid. Here sulfuric acid represents a weak diprotic acid. Again you want to make sure that your weak species, in this case the weak acid is greater in amount to ensure the creation of a buffer. So here let me put 2 molar, also a furus acid. Right, so just remember a buffer is a acid and base together, and these are the 3 specific ways to make any type of buffer.
3
example
Intro to Buffers Example
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2m
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Select all pair or pairs that could form a buffer solution. Remember, a buffer has a acid and a base, but there are 3 specific ways to make a buffer. One way to make a buffer is to have a weak acid plus its conjugate base or a conjugate acid plus its weak base. 2nd way to make it is to have a strong acid with a weak base. But in this instance, because it's strong and weak, the weak species needs to be greater in amount, greater in concentration. And then finally the last way to make one is to have a weak acid and a strong base. Again, the weak species needs to be greater in amount. So we take a look here, in the first one we have acetic acid, this is a weak acid, and hydrofluoric acid which is another weak acid. The only time a weak acid could help make a buffer is if that weak acid is with its conjugate base. Neither one of them is a conjugate of each other. Also, we could have a weak acid with a strong base. Yes, we have weak acids, but where's the strong base? Next, we have nitric acid and ammonia. Nitric acid represents a strong acid, ammonia represents a weak base. Here strong acid weak base is one of the 3 combinations. We're not talking about the amount of each, but we're assuming that the weak species is greater in amount, therefore a buffer would be created. So this is an answer. Next, we have hydrochloric acid which is a strong acid. Sodium Chloride has one less hydrogen than hydrochloric acid, so this is the conjugate base of the strong acid. Now, this does not mean it's a buffer because remember it's a weak acid plus its conjugate base, not a strong acid and its conjugate base. Next, we have potassium hydroxide which is a strong base, with hydrocyanic acid which is a weak acid. Again, this is one of the combinations that could create a buffer, so this is an answer. Then finally we have sodium bromide and sodium hydroxide. Well, sodium bromide here is a strong base, and the only way a strong base could help make a buffer is it linked up with a weak acid. NABR is basically a neutral ionic salt, it's not even a weak acid. So this could not work. So, out of my choices option B and D would be the pairs that could form a buffer solution.
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Problem
Problem
Which pairs of compounds are capable of making a buffer? Select all that apply.
a) 1.3 M LiOH and 1.7 M HCOOH c) 0.35 M CH3CO2H and 0.35 M NaOH
b) 0.784 M NH4+ and 0.800 M HClO4 d) 0.80 HNO3 and 0.15 MgO
A
1.3 M LiOH and 1.7 M HCOOH
B
0.784 M NH4+ and 0.800 M HClO4
C
0.35 M CH3CO2H and 0.35 M NaOH
D
0.80 HNO3 and 0.15 MgO
5
concept
Buffer Capacity
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45s
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When we refer to buffer capacity it's just the amount of acid or base that a buffer can neutralize before the pH of the solution start to noticeably change. Here we're going to say the larger the concentration of buffer components then the greater the buffer capacity, and we're going to say higher concentrations of weak acid and conjugate base remember this is the most common pair for buffer equals better buffer. Right? So basically we want more amounts of the base and acid components of a buffer, so that it can more effectively neutralize any additional acid or base added to it.
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example
Intro to Buffers Example
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Which of the following combinations would make a buffer with the greatest buffering capacity? Here we're dealing with 1 liter of solution. If we take a look in options a to d, we have different mole amounts of weak acid and conjugate base. For both a and b, we're dealing with chlorous acid, which is a weak acid, and we're dealing with sodium chloride, its conjugate base. For c and d, we're dealing with nitrous acid and potassium nitrite. If we take a look here, if we compare a and b to one another, we have point 25 moles versus point 35 moles for our weak acids, 0.20 moles of our conjugate base, 0.25 moles of our conjugate base. B has higher values for weak acid and conjugate base, so we'd have a greater buffering capacity. So B would be better than A. If we look at C, C also has weak acid and conjugate base, and the mole amounts are even larger. So option c would be better than option b. And then finally if we're comparing c and d, we have 0.35 moles versus 0.50 moles, 0.30 moles versus 0.48 moles. Option d will be the better answer since our weak acid and conjugate base have higher values, therefore giving them the greater buffering capacity. So here our final answer for this question will be option d.
7
concept
Buffer Range
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1m
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When taking into consideration a buffer, we must also look at buffer range. Here we're going to say that a buffer is effective as long as it has the right concentration ratio of weak species to its conjugate. Here for buffer range, we have a ratio of weak acid to conjugate base. It is effective if that ratio is a 10 to 1 ratio or a 1 to 10 ratio. Basically, at max, one of them can only be 10 times more than the other in terms of amount. If it goes beyond that range, it's no longer going to be an effective buffer. Here we're going to say a buffer is ideal or most effective when the concentration or amount of weak acid is equal to the amount of conjugate base. Here we're going to say the larger the difference in concentrations between weak species and its conjugate, then the less affected a buffer will be. So again, you always want to play around with that 10 to 1 ratio, or 1 to 10 ratio when comparing your weak acid to its conjugate. So to make sure that it will be as effective as possible.
8
example
Intro to Buffers Example
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2m
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Which of the following combinations would create the most effective buffer? So remember, with a buffer, we talk about buffer range. A buffer is effective as long as the ratio between weak acid and conjugate base is a 10 to 1 or 1 to 10 ratio, and we know that the most effective or ideal buffer is when the amount of weak acid is equal to the amount of conjugate base. So weak acid to conjugate base, if it can equal to 1, that's the best type of buffer. So what we're going to do here is we're going to figure out what the ratio is for each of the weak acid conjugate base pairs. The one that has a ratio closest to 1 would be most ideal and therefore most effective buffer. So if we take a look here, in all of these examples, what do we have? Well, we have here is Methylammonium ion. It is a positively charged amine, which remember positively charged amines are weak acids. And then methylamine is just its version with one less H plus, so this would be the conjugate base. All we're going to do here is we're going to take the ratios of each of them and see which one comes closest to 1. That would represent the most ideal buffer. So for a, the ratio of weak acid to conjugate base equals 1.0 over 1.2, which comes out to 0.83. For b, what do we have? We have weak acid 0.78 divided by conjugate base 1.3, so this comes out to 0.60. For c, we have 1.5 molar of our weak acid acid divided by 0.25 molar, so this comes out to 6. And then d, we have 6.8 molar of our weak acid, divided by 0.68 molar of our conjugate base, this comes out to 10. Based on these ratios, the one closest to 1 is option a. Option a will represent the pairing that is the most ideal buffer and therefore the most effective buffer. Now if they asked us the opposite, which of these options would be the least effective buffer, we'd say option D, because option D, we have such a vast difference in their concentrations. It's right on the on the edge of our buffer range. Right. So keep in mind for this particular one, the answer is option a.
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Problem
Problem
Determine which of the following actions will destroy a buffer composed of 0.50 L of 1.44 M H3PO4 and 0.60 L of 1.25 M NaH2PO4−.
a) Addition of 1.45 moles of KH2PO4
b) Addition of 0.85 moles of HCl
c) Addition of water
d) Addition of 0.30 moles of Ca(OH)2
e) Addition of 0.70 moles of HIO4
A
Addition of 1.45 moles of KH2PO4
B
Addition of 0.85 moles of HCl
C
Addition of water
D
Addition of 0.30 moles of Ca(OH)2
E
Addition of 0.70 moles of HIO4
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