8.7 pH and pKa
3 min read•january 9, 2023
Dylan Black
Jillian Holbrook
Dylan Black
Jillian Holbrook
There are many measures in AP Chemistry, especially relating to acids and bases and equilibrium. In this section, we will discuss the relationships between and , two of the most important measures!
‘p’ Notation
' is actually fairly simple and seen throughout acid-base chemistry (we have , , , , and many more!). 'p'-something is simply equal to the -log(something). For example, = -log(H+), and = -log(OH-). Similarly, = -log(Ka).
pKa and Acid Strength
An important use of is in describing relative to other acids. For example, if one acid has a of 3 and another has a of 2, we know that the acid with a of 2 is 10 times as acidic (note, however, that this does not mean that the is 10 times lower). Using ' gives us a and not a linear one.
Like , where a lower corresponds to a higher [H+], a lower implies a higher Ka. However, it is worth noting that a high does not imply basicity. Another note is that, like and , + = 14.
pH, pKa, and Buffers
and are also related to . As a reminder, a buffer is a mixture of an acid and its and is important because it is resistant to changes in . However, a question arises: when is the buffer the strongest?
The can be applied to find the of a buffer:
The strongest buffer occurs when the concentration of [A-] is equal to [HA]. In this case, = + log(1) ⇒ = . The relationship is vital, especially when looking at titration curves, because this same point occurs at the , implying that you have the at the .
Acid-Base Indicators
Finally, we will discuss . are a class of compounds that change color depending on the of the solution they are in. You may have used indicators in class during titrations to note when the of a titration occurs. Some examples of are , , and .
When choosing an acid-base indicator, you usually want to pick one in which your will end up in the , which is the plus or minus 1. While you will not need to memorize any indicators or their effective ranges on the exam, you may be asked to pick which one is the most effective for a certain experiment.
Let's see this concept with an FRQ from 2010:
We are given the following prompt:
In order for the indicator to be useful to us, we want it to change color at the for this titration. Looking at the graph, we see that the at the is 7. We also know this because it is a strong acid strong base titration. Therefore, we want to pick an indicator with a range closest to 7. This turns out to be , which is the correct answer.
Key Terms to Review (19)
Acid Strength
: Acid strength refers to the ability of an acid (a substance that donates hydrogen ions) to donate its protons or hydrogen ions. The stronger the acid, the more readily it gives up its proton.Acid-Base Indicators
: Acid-base indicators are substances that change color depending on whether they're in an acidic or basic environment.Bromothymol Blue
: Bromothymol blue is a pH indicator known for its color change from yellow to blue as the solution changes from acidic to basic.Buffers
: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.Conjugate Base
: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.Effective Range
: The effective range refers to the span within which a particular measurement tool can accurately measure. For indicators, this would be their color change range where they can accurately determine the pH.Equivalence Point
: The equivalence point in a titration is when exactly enough titrant has been added to react completely with the substance being analyzed.Half-Equivalence Point
: The half-equivalence point in a titration occurs when the amount of titrant added is exactly half what is needed to reach the equivalence point.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.Linear Scale
: A linear scale is one where equal distances on the scale represent equal differences in value. It's straightforward and direct - if you move from 1 to 2 on this type of scale, it represents the same change as moving from 2 to 3.Logarithmic Scale
: A logarithmic scale is a nonlinear scale used when there is a large range of quantities. In this scale, each increment on the axis increases by a factor of a fixed value, rather than by equal increments.Methyl Red
: Methyl red is another pH indicator used for identifying pH values on the lower end of the scale. It appears red in pH under 4.4 and yellow over 6.2.Optimal Buffer
: An optimal buffer is a solution that resists changes in pH when small amounts of an acid or a base are added to it. It's most effective when the pH is close to the pKa value of the buffering agent.p' Notation
: In chemistry, the 'p' notation is used to denote the negative logarithm of a quantity. It's often used in relation to acidity (pH), basicity (pOH), and equilibrium constants (pKa, pKb).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.Phenolphthalein
: Phenolphthalein is a chemical compound often used as an indicator in titrations. It is colorless in acidic solutions and turns pink in basic solutions.pKa
: The pKa value is used to express the strength of acids. It's defined as -log10 Ka where Ka is the acid dissociation constant.pKb
: pKb is a measure of basicity or alkalinity. It's defined as the negative logarithm (base 10) of base dissociation constant (Kb). A lower pKb value indicates a stronger base.pOH
: pOH is a measure of the concentration of hydroxide ions (OH-) in a solution. Like pH, it also uses a scale from 0 to 14 but inversely related to pH - lower pOH values indicate more basic solutions and higher values indicate more acidic solutions.8.7 pH and pKa
3 min read•january 9, 2023
Dylan Black
Jillian Holbrook
Dylan Black
Jillian Holbrook
There are many measures in AP Chemistry, especially relating to acids and bases and equilibrium. In this section, we will discuss the relationships between and , two of the most important measures!
‘p’ Notation
' is actually fairly simple and seen throughout acid-base chemistry (we have , , , , and many more!). 'p'-something is simply equal to the -log(something). For example, = -log(H+), and = -log(OH-). Similarly, = -log(Ka).
pKa and Acid Strength
An important use of is in describing relative to other acids. For example, if one acid has a of 3 and another has a of 2, we know that the acid with a of 2 is 10 times as acidic (note, however, that this does not mean that the is 10 times lower). Using ' gives us a and not a linear one.
Like , where a lower corresponds to a higher [H+], a lower implies a higher Ka. However, it is worth noting that a high does not imply basicity. Another note is that, like and , + = 14.
pH, pKa, and Buffers
and are also related to . As a reminder, a buffer is a mixture of an acid and its and is important because it is resistant to changes in . However, a question arises: when is the buffer the strongest?
The can be applied to find the of a buffer:
The strongest buffer occurs when the concentration of [A-] is equal to [HA]. In this case, = + log(1) ⇒ = . The relationship is vital, especially when looking at titration curves, because this same point occurs at the , implying that you have the at the .
Acid-Base Indicators
Finally, we will discuss . are a class of compounds that change color depending on the of the solution they are in. You may have used indicators in class during titrations to note when the of a titration occurs. Some examples of are , , and .
When choosing an acid-base indicator, you usually want to pick one in which your will end up in the , which is the plus or minus 1. While you will not need to memorize any indicators or their effective ranges on the exam, you may be asked to pick which one is the most effective for a certain experiment.
Let's see this concept with an FRQ from 2010:
We are given the following prompt:
In order for the indicator to be useful to us, we want it to change color at the for this titration. Looking at the graph, we see that the at the is 7. We also know this because it is a strong acid strong base titration. Therefore, we want to pick an indicator with a range closest to 7. This turns out to be , which is the correct answer.
Key Terms to Review (19)
Acid Strength
: Acid strength refers to the ability of an acid (a substance that donates hydrogen ions) to donate its protons or hydrogen ions. The stronger the acid, the more readily it gives up its proton.Acid-Base Indicators
: Acid-base indicators are substances that change color depending on whether they're in an acidic or basic environment.Bromothymol Blue
: Bromothymol blue is a pH indicator known for its color change from yellow to blue as the solution changes from acidic to basic.Buffers
: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.Conjugate Base
: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.Effective Range
: The effective range refers to the span within which a particular measurement tool can accurately measure. For indicators, this would be their color change range where they can accurately determine the pH.Equivalence Point
: The equivalence point in a titration is when exactly enough titrant has been added to react completely with the substance being analyzed.Half-Equivalence Point
: The half-equivalence point in a titration occurs when the amount of titrant added is exactly half what is needed to reach the equivalence point.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.Linear Scale
: A linear scale is one where equal distances on the scale represent equal differences in value. It's straightforward and direct - if you move from 1 to 2 on this type of scale, it represents the same change as moving from 2 to 3.Logarithmic Scale
: A logarithmic scale is a nonlinear scale used when there is a large range of quantities. In this scale, each increment on the axis increases by a factor of a fixed value, rather than by equal increments.Methyl Red
: Methyl red is another pH indicator used for identifying pH values on the lower end of the scale. It appears red in pH under 4.4 and yellow over 6.2.Optimal Buffer
: An optimal buffer is a solution that resists changes in pH when small amounts of an acid or a base are added to it. It's most effective when the pH is close to the pKa value of the buffering agent.p' Notation
: In chemistry, the 'p' notation is used to denote the negative logarithm of a quantity. It's often used in relation to acidity (pH), basicity (pOH), and equilibrium constants (pKa, pKb).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.Phenolphthalein
: Phenolphthalein is a chemical compound often used as an indicator in titrations. It is colorless in acidic solutions and turns pink in basic solutions.pKa
: The pKa value is used to express the strength of acids. It's defined as -log10 Ka where Ka is the acid dissociation constant.pKb
: pKb is a measure of basicity or alkalinity. It's defined as the negative logarithm (base 10) of base dissociation constant (Kb). A lower pKb value indicates a stronger base.pOH
: pOH is a measure of the concentration of hydroxide ions (OH-) in a solution. Like pH, it also uses a scale from 0 to 14 but inversely related to pH - lower pOH values indicate more basic solutions and higher values indicate more acidic solutions.
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