Fiveable
Fiveable

or

Log in

Find what you need to study


Light

Find what you need to study

8.2 pH and pOH of Strong Acids and Bases

4 min readjanuary 8, 2023

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

pH and pOH Explained

and are extremely important topics in Unit 8 and AP Chemistry as a whole. So then, what is ? is simply a measure of the of in a solution.

pH

Essentially, is measured by the of H+ ions, meaning it measures how acidic a solution is. A higher means a lower of H+, implying a more basic solution and vice versa (I know, that is really annoying). So, we get why there is an H in - H means hydrogen, and measures the of . What about the p? In chemistry, 'p' is a symbol that means . p(anything) = -log(anything). Thus, = -log([H+]).

For example, if we had a solution that had 0.01M H+, that would mean = -log(1 * 10^-2) = -(-2) = 2 (note that log() implies log base 10).

pOH

If = -log([H+]), then one can logically find that  = -log([OH-]) and is, in essence, the opposite of . Where is the of H+, is the of OH-. measures how basic a solution is. A low means a highly basic solution and vice versa.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-3dCUhwjByES3.jpg?alt=media&token=82858257-a259-4ed4-983d-a2b236fde64a

The Autoionization of Water

To elaborate, let us suggest we had the reaction OH- + H+ --> H2O, which can be described as the protonation of OH- or the . The aforementioned reaction serves as the net ionic for a . This reaction can further be described using by flipping the reaction to say that H2O ⇌ OH- + H+. The K value for this reaction is a constant called Kw. Kw = 1 * 10^-14.

Think about this reaction a little bit. From Kw that we were given before, we can write that Kw = [OH-][H+]. Therefore, 10^-14 = [OH-][H+]. Now, we can take the of both sides:

14 = -log([OH-][H+]) = -log([OH-]) + -log([H+])

From here, it is clear then that + = 14.

pH/pOH of Strong Acid and Strong Base Solutions

Finding the pH of Strong Acid/Base Solutions

For these examples, we will just be looking at since it is the most common measure of , but remember, can be easily calculated from this. When dealing with strong acids (and by extension strong bases), what is important is that these reactions go fully forwards. That is to say, they have a K value so high that to call it an is negligible. Therefore, all of the acid/base will .

Take a look at the following example:

Suppose we have a 1M solution of HCl. The dissolution of HCl looks like: HCl --> H+ + Cl- (we're gonna stick with the to make things simpler). Because this reaction goes completely forward, we know that 1M of HCl will dissolve into 1M of H+ and 1M of Cl-. To find , we take the -log([H+]).

= -log(1) = -0 = 0. Therefore a 1M solution of HCl has a of 0.

The same idea applies to strong bases. Suppose we had 1M NaOH. NaOH --> Na+ + OH-, so = -log([OH-]) = -log(1) = 0.

To find , we then plug our value into + = 14, which tells us that the of a 1M solution of NaOH is 14.

The List of Strong Acids

There are seven strong acids you need to memorize for AP Chemistry. Luckily, they're not that bad, and they become second nature after a while through practice:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-dFBEEQda3St5.png?alt=media&token=20e41fc4-70f5-46d1-a061-8b498b575e42

These acids completely ionize in aqueous solutions to produce . As such, the of H3O+ in a strong acid solution is equal to the initial of the strong acid, and thus the of the strong acid solution is easily calculated.

Key Terms to Review (15)

Acidity

: Acidity refers to how much acid is present in a substance, or its ability to donate protons (H+ ions).

Arrhenius Definition of Acids

: According to Arrhenius, an acid is any substance that when dissolved in water increases the concentration of hydrogen ions (H+).

Autoionization of Water

: Autoionization of water is the process where two water molecules react to produce one hydronium ion (H3O+) and one hydroxide ion (OH-).

Basicity

: Basicity refers to how much base is present in a substance, or its ability to accept protons (H+ ions).

Concentration

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

Dissociate

: Dissociation in chemistry refers to the process where compounds split into smaller particles, usually ions.

Equilibrium

: Equilibrium refers to the state in which both reactants and products are present in concentrations which have no further tendency to change over time. It's when forward and reverse reactions occur at equal rates so there's no net change observed.

Hydrogen Ions

: Hydrogen ions (H+) are single protons with no electrons, meaning they carry a positive charge.

Hydronium Ions

: Hydronium ions are formed when an acid donates proton(s) to water molecules. They are represented as H3O+.

Kw (Water Dissociation Constant)

: The water dissociation constant, Kw, is the equilibrium constant for the self-ionization of water into hydrogen ions and hydroxide ions. It's a measure of the concentration of these ions in pure water at a specific temperature.

Negative Log

: In chemistry, negative log often refers to calculating pH values by taking the negative logarithm base 10 (-log) of the molar concentration of H+ 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.

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.

Protons

: Protons are positively charged subatomic particles found within atomic nuclei.

Strong Acid-Strong Base Interaction

: This refers to the neutralization reaction between strong acids and strong bases, resulting in salt and water.

8.2 pH and pOH of Strong Acids and Bases

4 min readjanuary 8, 2023

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

Dylan Black

Dylan Black

Jillian Holbrook

Jillian Holbrook

pH and pOH Explained

and are extremely important topics in Unit 8 and AP Chemistry as a whole. So then, what is ? is simply a measure of the of in a solution.

pH

Essentially, is measured by the of H+ ions, meaning it measures how acidic a solution is. A higher means a lower of H+, implying a more basic solution and vice versa (I know, that is really annoying). So, we get why there is an H in - H means hydrogen, and measures the of . What about the p? In chemistry, 'p' is a symbol that means . p(anything) = -log(anything). Thus, = -log([H+]).

For example, if we had a solution that had 0.01M H+, that would mean = -log(1 * 10^-2) = -(-2) = 2 (note that log() implies log base 10).

pOH

If = -log([H+]), then one can logically find that  = -log([OH-]) and is, in essence, the opposite of . Where is the of H+, is the of OH-. measures how basic a solution is. A low means a highly basic solution and vice versa.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-3dCUhwjByES3.jpg?alt=media&token=82858257-a259-4ed4-983d-a2b236fde64a

The Autoionization of Water

To elaborate, let us suggest we had the reaction OH- + H+ --> H2O, which can be described as the protonation of OH- or the . The aforementioned reaction serves as the net ionic for a . This reaction can further be described using by flipping the reaction to say that H2O ⇌ OH- + H+. The K value for this reaction is a constant called Kw. Kw = 1 * 10^-14.

Think about this reaction a little bit. From Kw that we were given before, we can write that Kw = [OH-][H+]. Therefore, 10^-14 = [OH-][H+]. Now, we can take the of both sides:

14 = -log([OH-][H+]) = -log([OH-]) + -log([H+])

From here, it is clear then that + = 14.

pH/pOH of Strong Acid and Strong Base Solutions

Finding the pH of Strong Acid/Base Solutions

For these examples, we will just be looking at since it is the most common measure of , but remember, can be easily calculated from this. When dealing with strong acids (and by extension strong bases), what is important is that these reactions go fully forwards. That is to say, they have a K value so high that to call it an is negligible. Therefore, all of the acid/base will .

Take a look at the following example:

Suppose we have a 1M solution of HCl. The dissolution of HCl looks like: HCl --> H+ + Cl- (we're gonna stick with the to make things simpler). Because this reaction goes completely forward, we know that 1M of HCl will dissolve into 1M of H+ and 1M of Cl-. To find , we take the -log([H+]).

= -log(1) = -0 = 0. Therefore a 1M solution of HCl has a of 0.

The same idea applies to strong bases. Suppose we had 1M NaOH. NaOH --> Na+ + OH-, so = -log([OH-]) = -log(1) = 0.

To find , we then plug our value into + = 14, which tells us that the of a 1M solution of NaOH is 14.

The List of Strong Acids

There are seven strong acids you need to memorize for AP Chemistry. Luckily, they're not that bad, and they become second nature after a while through practice:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-dFBEEQda3St5.png?alt=media&token=20e41fc4-70f5-46d1-a061-8b498b575e42

These acids completely ionize in aqueous solutions to produce . As such, the of H3O+ in a strong acid solution is equal to the initial of the strong acid, and thus the of the strong acid solution is easily calculated.

Key Terms to Review (15)

Acidity

: Acidity refers to how much acid is present in a substance, or its ability to donate protons (H+ ions).

Arrhenius Definition of Acids

: According to Arrhenius, an acid is any substance that when dissolved in water increases the concentration of hydrogen ions (H+).

Autoionization of Water

: Autoionization of water is the process where two water molecules react to produce one hydronium ion (H3O+) and one hydroxide ion (OH-).

Basicity

: Basicity refers to how much base is present in a substance, or its ability to accept protons (H+ ions).

Concentration

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

Dissociate

: Dissociation in chemistry refers to the process where compounds split into smaller particles, usually ions.

Equilibrium

: Equilibrium refers to the state in which both reactants and products are present in concentrations which have no further tendency to change over time. It's when forward and reverse reactions occur at equal rates so there's no net change observed.

Hydrogen Ions

: Hydrogen ions (H+) are single protons with no electrons, meaning they carry a positive charge.

Hydronium Ions

: Hydronium ions are formed when an acid donates proton(s) to water molecules. They are represented as H3O+.

Kw (Water Dissociation Constant)

: The water dissociation constant, Kw, is the equilibrium constant for the self-ionization of water into hydrogen ions and hydroxide ions. It's a measure of the concentration of these ions in pure water at a specific temperature.

Negative Log

: In chemistry, negative log often refers to calculating pH values by taking the negative logarithm base 10 (-log) of the molar concentration of H+ 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.

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.

Protons

: Protons are positively charged subatomic particles found within atomic nuclei.

Strong Acid-Strong Base Interaction

: This refers to the neutralization reaction between strong acids and strong bases, resulting in salt and water.


© 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.


© 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.