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8.10 Buffer Capacity

4 min readjanuary 9, 2023

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Welcome to the last section of Unit 8! This section is a relatively simple and short one all about , which helps us gauge the effectiveness of . For a quick reminder, are important because they are resistant to changes in .

However, are not infinitely resistant. Eventually, the buffer will weaken and succumb to the /base being added. This is why, despite there being in your bloodstream, chugging hydrochloric is a very bad idea. Seriously, don't do it!

helps us see how much /base one can add until there is a significant change in .

Describing Buffer Capacity

As said by the , the of a buffer is defined by the ratio of the concentrations of the to the , or in math terms [A-]/[HA]. The capacity of a buffer is determined by the magnitudes of these concentrations.

What do we mean when we refer to ? Essentially, the of each describes how large the concentrations are. A of 5M would have a higher than a 0.5M solution. The more concentrated the and , the stronger the buffer is at reducing changes! There is more and to be resistant to / in a similar volume.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-JFUOtGB741sL.gif?alt=media&token=7ada2b5f-146c-4220-bb6b-653388719149

Image From ChemCollective

Example Problem: Identifying the Stronger Buffer

To practice applying , try thinking about two separate buffer systems, one with 5M and 5M and another with 0.05M and 0.05M . Because the ratios of the to the are the same, both of these will have the same (=4.74). However, our question asks us the following:

After HCl is added to each buffer system, the first one has a resulting of 4.74, and the second one has a resulting of 4.56. Which one has the better buffering capacity and why?

We can see that in the first buffer, the remained relatively unchanged, whereas, in the second, the dropped. Because the first system's remained constant, the first buffer is more effective at resisting changes and, therefore, has a better buffering capacity. The of the of both the and the is higher in the first buffer compared to the second, implying the first buffer will also have a stronger .

Practice Multiple Choice Questions

The College Board likes to use specifically on multiple choice questions because, unlike many other questions relating to , questions about are very often qualitative. That means your answer will relate to some sort of non-numerical conclusion based on the information you are given.

They could ask you to identify the stronger buffer, like in the last question, or ask about a change in a system and how this may affect the .

Here is another example:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-UPISRg0lqvyV.png?alt=media&token=cc937bdc-81b8-48a2-bace-8da6cc11f208

Image From Abigail Giordano

This problem gives us a BUNCH of information and can be really overwhelming at first. However, by breaking it down piece by piece, the problem is more approachable.

We know that a student is creating a buffer of (CH3COOH, also stated as HC2H3O2) and since a mixes with its . Now we can examine the numbers. The first set of numeric information indicates that the student wants to mix 250 mL of 0.100M with 500 mL of 0.440M .

Yikes! The student makes an error. They choose an with half the and a with half of the volume. For , the volume remains at 250mL, but the is now 0.0500M. Meanwhile, the of the stays at 0.440M, but the volume falls from 500mL to 250mL. The problem wants to know the ramifications of these mistakes.

Before looking at the answer choices, what changed? In both sets (the and the ), the number of is halved. Therefore, we will have half the number of of each species in our buffer.

Again, before looking at any answer choices, how is the buffer impacted? As we discussed earlier, a lower number of of each reactant in the buffer creates less resistance to changes in . In other words, the lowers.

Which answer choice matches that concept?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-qpeKcnEcwrKR.png?alt=media&token=682ce6fc-b485-4ce3-862c-3d5ba732873f

Image From Abigail Giordano

If we read the answer choices, we see that what we just described fits answer A. You may be tempted to pick B, but remember that is determined by the number of of the and the

Key Terms to Review (15)

Acetic Acid

: Acetic acid is an organic compound with the formula CH3COOH. It's what gives vinegar its sour taste and pungent smell.

Acid

: An acid is a substance that donates protons (hydrogen ions) when dissolved in water, resulting in an acidic pH.

Buffer Capacity

: Buffer capacity refers to the amount of acid or base that can be added to a buffer solution before its pH starts to change significantly.

Buffers

: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.

Concentration

: In chemistry, concentration refers to the amount of a substance per defined space. It's usually measured in terms of mass per volume.

Conjugate Base

: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.

Henderson-Hasselbalch Equation

: The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a solution given the pKa (acid dissociation constant) and the ratio of the concentrations of the acid and its conjugate base.

Magnitude

: In chemistry, magnitude refers to the size or extent of something. It's often used to describe the amount or degree of a particular property or characteristic.

Molarity

: Molarity is a measure of the concentration of a solute in a solution, or of any chemical species in terms of amount of substance in a given volume.

Moles

: A mole is unit for measuring matter at atomic level. One mole contains exactly 6.02214076×10²³ particles, such as atoms, molecules, or ions.

pH

: pH is a measure of the acidity or alkalinity of a solution. It's calculated as the negative logarithm (base 10) of the concentration of hydrogen ions in a solution.

Sodium Acetate

: Sodium acetate is a sodium salt of acetic acid and has the chemical formula C2H3NaO2. It's often used in heating pads, hand warmers, and hot ice.

Strong Acids

: Strong acids are substances that completely ionize in water, releasing all their hydrogen ions (H+) into the solution.

Strong Bases

: Strong bases are substances that fully dissociate in water and release hydroxide ions (OH-) into the solution.

Weak Acid

: A weak acid is one that does not completely dissociate into its ions in water, meaning only some of its molecules donate protons when placed in water.

8.10 Buffer Capacity

4 min readjanuary 9, 2023

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Welcome to the last section of Unit 8! This section is a relatively simple and short one all about , which helps us gauge the effectiveness of . For a quick reminder, are important because they are resistant to changes in .

However, are not infinitely resistant. Eventually, the buffer will weaken and succumb to the /base being added. This is why, despite there being in your bloodstream, chugging hydrochloric is a very bad idea. Seriously, don't do it!

helps us see how much /base one can add until there is a significant change in .

Describing Buffer Capacity

As said by the , the of a buffer is defined by the ratio of the concentrations of the to the , or in math terms [A-]/[HA]. The capacity of a buffer is determined by the magnitudes of these concentrations.

What do we mean when we refer to ? Essentially, the of each describes how large the concentrations are. A of 5M would have a higher than a 0.5M solution. The more concentrated the and , the stronger the buffer is at reducing changes! There is more and to be resistant to / in a similar volume.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-JFUOtGB741sL.gif?alt=media&token=7ada2b5f-146c-4220-bb6b-653388719149

Image From ChemCollective

Example Problem: Identifying the Stronger Buffer

To practice applying , try thinking about two separate buffer systems, one with 5M and 5M and another with 0.05M and 0.05M . Because the ratios of the to the are the same, both of these will have the same (=4.74). However, our question asks us the following:

After HCl is added to each buffer system, the first one has a resulting of 4.74, and the second one has a resulting of 4.56. Which one has the better buffering capacity and why?

We can see that in the first buffer, the remained relatively unchanged, whereas, in the second, the dropped. Because the first system's remained constant, the first buffer is more effective at resisting changes and, therefore, has a better buffering capacity. The of the of both the and the is higher in the first buffer compared to the second, implying the first buffer will also have a stronger .

Practice Multiple Choice Questions

The College Board likes to use specifically on multiple choice questions because, unlike many other questions relating to , questions about are very often qualitative. That means your answer will relate to some sort of non-numerical conclusion based on the information you are given.

They could ask you to identify the stronger buffer, like in the last question, or ask about a change in a system and how this may affect the .

Here is another example:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-UPISRg0lqvyV.png?alt=media&token=cc937bdc-81b8-48a2-bace-8da6cc11f208

Image From Abigail Giordano

This problem gives us a BUNCH of information and can be really overwhelming at first. However, by breaking it down piece by piece, the problem is more approachable.

We know that a student is creating a buffer of (CH3COOH, also stated as HC2H3O2) and since a mixes with its . Now we can examine the numbers. The first set of numeric information indicates that the student wants to mix 250 mL of 0.100M with 500 mL of 0.440M .

Yikes! The student makes an error. They choose an with half the and a with half of the volume. For , the volume remains at 250mL, but the is now 0.0500M. Meanwhile, the of the stays at 0.440M, but the volume falls from 500mL to 250mL. The problem wants to know the ramifications of these mistakes.

Before looking at the answer choices, what changed? In both sets (the and the ), the number of is halved. Therefore, we will have half the number of of each species in our buffer.

Again, before looking at any answer choices, how is the buffer impacted? As we discussed earlier, a lower number of of each reactant in the buffer creates less resistance to changes in . In other words, the lowers.

Which answer choice matches that concept?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-qpeKcnEcwrKR.png?alt=media&token=682ce6fc-b485-4ce3-862c-3d5ba732873f

Image From Abigail Giordano

If we read the answer choices, we see that what we just described fits answer A. You may be tempted to pick B, but remember that is determined by the number of of the and the

Key Terms to Review (15)

Acetic Acid

: Acetic acid is an organic compound with the formula CH3COOH. It's what gives vinegar its sour taste and pungent smell.

Acid

: An acid is a substance that donates protons (hydrogen ions) when dissolved in water, resulting in an acidic pH.

Buffer Capacity

: Buffer capacity refers to the amount of acid or base that can be added to a buffer solution before its pH starts to change significantly.

Buffers

: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.

Concentration

: In chemistry, concentration refers to the amount of a substance per defined space. It's usually measured in terms of mass per volume.

Conjugate Base

: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.

Henderson-Hasselbalch Equation

: The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a solution given the pKa (acid dissociation constant) and the ratio of the concentrations of the acid and its conjugate base.

Magnitude

: In chemistry, magnitude refers to the size or extent of something. It's often used to describe the amount or degree of a particular property or characteristic.

Molarity

: Molarity is a measure of the concentration of a solute in a solution, or of any chemical species in terms of amount of substance in a given volume.

Moles

: A mole is unit for measuring matter at atomic level. One mole contains exactly 6.02214076×10²³ particles, such as atoms, molecules, or ions.

pH

: pH is a measure of the acidity or alkalinity of a solution. It's calculated as the negative logarithm (base 10) of the concentration of hydrogen ions in a solution.

Sodium Acetate

: Sodium acetate is a sodium salt of acetic acid and has the chemical formula C2H3NaO2. It's often used in heating pads, hand warmers, and hot ice.

Strong Acids

: Strong acids are substances that completely ionize in water, releasing all their hydrogen ions (H+) into the solution.

Strong Bases

: Strong bases are substances that fully dissociate in water and release hydroxide ions (OH-) into the solution.

Weak Acid

: A weak acid is one that does not completely dissociate into its ions in water, meaning only some of its molecules donate protons when placed in water.


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© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.