The capacity of a buffer to neutralize added acid or base depends on the concentrations of HA and A⁻ in solution. For a given ratio of [HA] to [A⁻], the greater the concentrations, the higher the overall buffer capacity. When [HA] is greater than [A⁻], the capacity is higher for added base than acid. When [A⁻] is greater than [HA], the capacity is higher for added acid than base. Created by Jay.
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- how would you go about determining the capacity of a basic buffer?(2 votes)
- Buffer capacity is defined as the number of moles of acid or base that have to be added to 1 liter to cause its pH to change by 1 unit.
As a formula this is: B = n/ΔpH, where B is buffer capacity (which is unitless), n is the number of moles of acid or base that was added to the buffer per liter of the buffer, and ΔpH is the pH difference between the initial buffer’s pH and the pH after the addition of acid or base to the buffer.
A larger B value would mean the buffer has the capacity to neutralize larger amounts of acid or base before it no longer functions.
Hope that helps.(3 votes)
- When using the ICF table, why does the conjugate base CH3COO- initially have a concentration of 0.250 and 0.0250? Shouldnt it be 0 on the right side?(1 vote)
- a good buffer should contain approximately the same amount of conjugate acid/base as weak acid/base. in this vid, jay started out with equal amounts of conjugate base and weak acid and so in all the tables, there are equal amounts there. it is not zero because there should already be an equal amount of conjugate base in order for the buffer to be considered good. see link below for vid on properties of buffers:
hope this helps:)(2 votes)
- where sis 4.70 com from when doing ph(1 vote)
- That's the pKa of acetic acid, or the -log(Ka), which is a measure of how strong an acid is. A pKa is a constant specific to a chemical and can be found in most chemistry textbook's data sections. In a problem/exam situation they're almost guaranteed to provide this information.
Hope that helps.(1 vote)
- [Instructor] Buffer capacity refers to the amount of acid or base a buffer can neutralize before the pH changes by a large amount. An increased buffer capacity means an increased amount of acid or base neutralized before the pH changes dramatically. Let's compare two buffers; buffer solution one and buffer solution two, and see which one has the higher buffer capacity. Buffer solution one has a concentration of acidic acid of 0.250 molar and a concentration of acetate anion also 0.250 molar. Buffer solution two also consists of acidic acid and the acetate anion. However, in this case, both concentrations are 0.0250 molar. So buffer solution one has a higher concentration of both acidic acid and the acetate anion. Let's calculate the initial pH of both buffer solutions using the Henderson-Hasselbalch equation. In the Henderson-Hasselbalch equation, the pH of the solution is equal to the pka of the weak acid, which for both buffers is a acidic acid plus the log of the concentration of the conjugate base divided by the concentration of the weak acid. In this case, the conjugate base is the acetate anion, so the concentration of the acetate anion divided by the concentration of acidic acid. For buffer solution number one, the concentration of the acetate anion is equal to the concentration of acidic acid. Therefore, the ratio of their concentrations is equal to one. And it's the same idea for buffer solution number two, the concentration of the acetate anion is equal to the concentration of acidic acid. Therefore, the ratio of their concentrations is also equal to one for buffer two. Since the ratio of the concentrations is equal to one, the log of one is equal to zero, and the pka value of acidic acid at 25 degrees Celsius is equal to 4.74. So the pH of both buffer solutions is equal to 4.74 plus zero, or just 4.74. So we're starting with two buffer solutions, each at a pH of 4.74, and to those buffer solutions, we're gonna add 0.0200 moles of hydroxide anions. And by calculating the pH change after adding the hydroxide anions, we'll be able to see which buffer has the higher buffer capacity. The added hydroxide anions will be neutralized by the weak acid that's present in the buffer system. So that's acidic acid. So acidic acid is going to react with the hydroxide anions. And to make the math easier, we're gonna assume that the total volume of the buffer solutions is equal to one that liter both before and after the addition of the base. So if the total volume of the solution is one liter and the concentration of acidic acid is equal to 0.250 molar in buffer one, that means there's 0.250 moles of acidic acid in the buffer. So 0.250 moles of a acidic acid will react with the 0.0200 moles of hydroxide anions that we're adding to buffer solution one. Let's calculate the pH of buffer solution one after the addition of the hydroxide anions. The hydroxide anions react with a acidic acid to form water and the acetate anion. In buffer solution one, the initial moles of both acidic acid and the acetate anion are both 0.250. And to that buffer solution we're adding 0.0200 moles of hydroxide anions. To help us find the final moles of acidic acid and the acetate anion, we're gonna use an ICF table, where I is the initial amount of moles, C is the change in moles and F as the final amount of moles. For this reaction, hydroxide anions are the limiting reactant. So we're gonna use up all of the 0.0200 moles of hydroxide anions, and we're left with nothing. Since the mole ratio of hydroxide anions to acidic acid is a one-to-one mole ratio, we're also gonna use up 0.0200 moles of acidic acid, which leaves us with 0.230 moles of acidic acid. And for the acetate anion, there's also a coefficient of one in the balanced equation. So for losing 0.0200 moles for the reactants, we're gaining 0.0200 moles for the acetate anion, which gives us a final amount of moles of the acetate anion of 0.270. Now that we have our final moles, we're ready to calculate the pH of the buffer solution. So the pH is equal to the pka value, which for a acidic acid is 4.74, plus the log of the ratio of the concentrations. And we could put in the concentrations, but since concentration or molarity is equal to moles divided by liters, a ratio of the concentrations would just have the volume cancel out because it's the same for both our conjugate base and our weak acid. So a ratio of the moles is the same thing as a ratio of the concentrations in the Henderson-Hasselbalch equation. And we can get our moles directly from our ICF table. So the moles of the acetate anion are equal to 0.270, and the moles of a acidic acid is equal to 0.230. And when we solve for the pH, we find the pH of the solution is equal to 4.81. So buffer one started at a pH of 4.74, and after the addition of the hydroxide anions, the pH rose a little bit to 4.81, however, that's a relatively small change in the pH. So buffer one did a pretty good job of resisting a large change in pH. Next, let's calculate the pH of buffer two after the addition of the hydroxide anions. So the initial moles of a acidic acid in buffer two is equal to 0.0250 moles, which is the same number of moles as the acetate anion, so 0.0250, and the hydroxide anions that we add is equal to 0.0200 moles. Once again, hydroxide anions are the limiting reactant, so all of the hydroxide anions are used up and we're left with zero moles. Since the mole ratio of hydroxide anions to acidic acid is one to one, we use up the same amount of acidic acid, 0.0200 moles, and we're left with 0.0050 moles of a acidic acid after the neutralization. And since there's a one as a coefficient in front of the acetate anion, the acetate anion is going to gain 0.0200 moles for a final moles of 0.0450. Next, we can calculate the pH of the solution using the Henderson-Hasselbalch equation. So the pH is equal to the pka value of acidic acid of 4.74, plus the log of the concentration of the conjugate base divided by the concentration of the weak acid. And since we can substitute moles for that, we can grab those from our ICF table and plug in the moles of acetate anion and the moles of acidic acid, and then solve for the pH. When we solve for the pH, we find that the pH of buffer solution two is equal to 5.69. So buffer solution two started at pH of 4.74, and after the addition of the hydroxide anions, the pH rose to 5.69. That's a relatively large increase in the pH of the solution. So going back to buffer solution one, the initial pH was 4.74, and the pH rose to 4.81 upon the addition of the base. For buffer solution two, we started at 4.74 and the pH rose to 5.69. Therefore buffer solution one had a higher capacity to neutralize the added base. And so we say that buffer solution one has the higher buffer capacity. And since buffer solution two had the more dramatic change in pH upon the addition of the same amount of base, buffer two has a decreased capacity to neutralize the base compared to buffer one. So we say that buffer two has a decreased buffer capacity. Remember that the only difference between these two buffers was that buffer one had a higher concentration of acidic acid and the acetate anion. Therefore the higher the concentration of the weak acid and the conjugate base, the higher the buffer capacity. We just looked at two buffer solutions in which the concentrations of weak acid and conjugate base were equal. However, a buffer solution doesn't have to start with equal concentrations of the weak acid and its conjugate base. For example, when the concentration of weak acid is greater than the concentration of conjugate base, the buffer has a higher capacity for added base than added acid. And when the concentration of conjugate base is greater than the concentration of weak acid, the buffer has a higher capacity for added acid than added base. As an example, let's think about blood, which has a pH of 7.4. The major buffer system used to control the pH of blood is the carbonic acid bicarbonate buffer system. In blood, the concentration of bicarbonate is greater than the concentration of carbonic acid. And since bicarbonate is the conjugate base to carbonic acid, the concentration of the conjugate base is greater than the concentration of the weak acid. And therefore the buffer system has a higher capacity for added acid than added base. The reason why the buffer system needs to have a higher capacity for added acid is because the products of metabolism that enter the bloodstream are mostly acidic and therefore the bicarbonate anion can react with those acidic products and neutralize them. Therefore, the buffer system is able to resist large changes to the pH of blood.