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pH Scale

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pH Scale

Have you ever taken probiotic supplements? Probiotics contain different types of live microorganisms that offer health benefits to our bodies, such as enhancing or restoring our microbiome health. One of the bacteria found in probiotics is called Lactobacillus.

Lactobacillus bacteria can be found as a part of the normal flora of the stomach. Since the stomach has low pH of around 1-3, we can say that Lactobacillus bacteria thrive in acidic environments. Substances containing a low pH are considered acidic, and a pH scale is used to show the degree of acidity or alkalinity of substances.

  • First, we will talk about the definition of pH.
  • Then, we will take a look at the differences in pH between acids and bases.
  • Next, we look at the pH scale ranges.
  • Lastly, we will learn how to calculate pH.

Definition of pH

First of all, we need to define pH.

pH is a measure of the hydrogen ion concentration of a solution.

But, what exactly does this mean? We need to go back and refresh the concept of Brønsted-Lowry acids and bases.

A Brønsted-Lowry acid is a proton (hydrogen ion) donor. A Brønsted-Lowry base is a proton (hydrogen ion) acceptor.

So, pH is a measurement of the concentration of hydrogen ions in a solution.

Scientists use a pH scale to show whether a solution is acidic or basic, depending on the concentration of hydrogen ions! The pH scale is considered a logarithmic scale. This means that decreasing or decreasing one pH unit will change the concentration by a tenfold!

pH Scale: Acids and Bases

Apart from the Brønsted-Lowry definition of acids and bases, acids and bases have other definitions. The first person to ever classify acids and bases was Svante Arrhenius, and he came up with the following definitions:

Arrhenius acids are substances that dissociate in water and produce H+ ions. Arrhenius bases are substances that dissociate in water and produce OH- ions.

Acids are proton donors, and they dissociate into negative ions and positive hydrogen ions (H+). For example, when hydrogen chloride (HCl) is dissolved in water, HCl undergoes ionization and produces H+ ions and Cl- ions.

pH scale HCl in water StudySmarterHCl in water, StudySmarter Originals.

Bases, on the other hand, dissociate to form positive ions and negative hydroxide ions (OH-). For example, when potassium hydroxide (KOH) is added to water, KOH ionizes to form OH- ions and K+ ions.

pH scale KOH in water StudySmarterKOH in water, StudySmarter Originals.

Strong Acids and Bases

According to Arrhenius definition, acids are proton donors in H2O, so they increase the concentration of H+ ions in aqueous solution. Strong acids are acids that completely dissociate in water. So, if we add strong acid to water, the following equation arises:

Similarly, strong bases are bases that dissociate completely in water. Dissolving a strong base in water can be represented with this equation:

If you want to know more about strong acids and bases, read "Acids and Bases" and "pH and pKa".

pH Scale: Range

The pH scale looks like this:

pH scale pH scale chart StudySmarterpH Scale, StudySmarter Originals.

Notice that the pH ranges from 0 to 14. Sometimes pH might run even outside this range, but it is very rare. Yes, negative pH is a thing. Solutions that contain a hydrogen ion concentration equivalent to a pH of less than 7 are considered acidic, whereas a pH greater than 7 is considered basic (also known as alkaline). When a substance contains a pH of 7, it is considered neutral.

A neutral solution is a solution that has equal concentrations of H+ and OH- ions.

But, where are strong acids and strong bases found in the pH scale? Strong acids are found in the pH range between 0 and 1, while strong bases are on the other extreme, having a pH of 14. This is due to the concentration of hydrogen ions in the solution.

  • Strong acids have a high concentration of H+, whereas strong bases have a low concentration of H+ ions in solution. The higher the [H+], the more acidic a solution is!

Danish chemist Soren Peder Lauritz Sorensen was the person who invented the pH scale. The pH scale was invented in 1909 as a way to help Sorensen control the acidity of his artisanal beers. He wanted to be able to control acidity in order to prevent the growth of undesirable bacteria.

An interesting fact about pH is that although we know that the H stands for hydrogen, no one is completely sure about the meaning of p.

pH Scale: Formula to calculate pH

The pH scale is considered a logarithmic scale corresponding to the concentration of protons (H+ ions) in aqueous solutions. So, pH is the negative log of [H+] and can be calculated using the following formula:

Some textbooks use [H3O+] to refer to the concentration of protons instead of [H+]. They mean the same thing, so don't feel confused if you encounter this difference!

Let's look at an example!

Find the pH of a solution where [H+] =.

We can use the pH formula above to calculate pH:

Keep in mind that pH is always given to two decimal places, and it also has no units!

Now, If we rearranged the pH formula, we could also use it to find the concentration of hydrogen ions in a solution!

Let's solve another example!

Calculate the mass (in grams) of HBr (strong acid) needed to mix into 200 mL of water in order to make a solution with a pH of 3.01?

We need to use the equation above to find the concentration of hydrogen ions in the solution. Then, we use that value to find the mass of HBr.

First, find the hydrogen ion concentration using the value for pH given.

Next, we need to convert mL to L:

Now, we can calculate the number of moles of HBr (solute) by rearranging the following formula:

Finally, we can use the number of moles to solve for the mass of HBr:

Because strong acids and bases dissociate completely in solution, we can say that the concentration of the strong acid or base will equal the concentration of the H+ or OH- in solution. So, to calculate the pH of strong acids and strong bases, we can use the pH equation we just learned!

Examples Involving pH Scale

If you had to guess, where do you think you would find the following substances on the pH scale?

  • Lemon juice
  • Milk
  • Baking soda
  • Bleach

We would expect Lemon juice to have a low pH, milk to be slightly acidic compared to pure water (pH 7), baking soda to be somewhat alkaline and bleach to have a high pH!

pH Scale pH of common substances shown on the pH scale StudySmarterpH of commons substances, StudySmarter Originals.

Did you know that a digital pH meter can be used to electronically measure the pH of a substance? The digital pH meter has an electronic probe that works by detecting the number of H+ ions in a solution. The more hydrogen ions, the lower the pH!

Now, you should be more familiar with the pH scale and how to find the pH of different substances!

pH Scale - Key takeaways

  • pH is referred to as a measure of the hydrogen ion concentration of a solution.
  • A Bronsted-Lowry acid is a proton (hydrogen ion) donor.
  • A Bronsted-Lowry base is a proton (hydrogen ion) acceptor
  • A pH scale is used to show the degree of acidity or alkalinity of substances.
  • To calculate pH from [H+] we can use the following formula:

References:

AP Chemistry course and exam description, effective fall 2020. (n.d.). Retrieved April 8, 2022, from https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf?course=ap-chemistry.

Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.

Swanson, J. W. (2020). Everything you need to Ace Chemistry in one big fat notebook. Workman Pub.

The Princeton Review: Cracking the AP Chemistry exam, 2020. (2019). New York, NY: Penguin Random House.

Frequently Asked Questions about pH Scale

The pH scale shows how acidic or alkaline an aqueous solution is. Knowing the pH of a solution is very important. For example, doctors can use pH to help them diagnose medical conditions. 


The pH scale ranges from 1 to 14 because the pH of aqueous solutions  usually falls within this range. However, in the lab it is common to talk about pH of 0 or some cases even negative pH-es (or above 15 for that matter) are possible.

To read a pH scale you must be familiar with the pH ranges, acidity, and alkalinity. Substances that contain a pH of less than 7 are considered acidic, whereas a pH greater than 7 is considered basic (also known as alkaline). When a substance contains a pH of 7, it is considered neutral.

The pH scales measure the degree of acidity and alkanility of aqueous solutions. Substances that contain a pH of less than 7 are considered acidic, whereas a pH greater than 7 is considered basic (also known as alkaline). When a substance contains a pH of 7, it is considered neutral.

The pH scale is used to show how acidic or basic (alkaline) an aqueous solution is.

Final pH Scale Quiz

Question

pH is referred to as a measure of the ______ concentration of a solution.

Show answer

Answer

hydrogen ion

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Question

A Bronsted-Lowry acid is a proton _____.

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Answer

donor

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Question

A Bronsted-Lowry base is a proton  _____.


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Answer

acceptor

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Question

True or false: a pH scale is used to show whether a solution is acidic or basic depending on the concentration of hydrogen ions.

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Answer

True

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Question

Arrhenius ____ are substances that dissociate in water and produce H+ ions.

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Answer

acids

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Question

Arrhenius ____ are substances that dissociate in water and produce OH- ions.

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Answer

bases

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Question

True or false: strong acids and bases partially dissociate in solution.

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Answer

False

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Question

The pH ranges from _____

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Answer

0 - 14

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Question

Substances that contain a pH of less than 7 are considered _____

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Answer

Acidic

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Question

Substances that have a pH greater than 7 are _______.

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Answer

Basic

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Question

When a substance contains a pH of 7, it is considered  _____.

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Answer

Neutral

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Question

A neutral solution is a solution that has ______ concentrations of H+ and OH- ions. 


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Answer

equal

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Question

Strong acids have a _____ concentration of H+ ions in solution.


Strong bases have a ____ concentration of H+ ions in solution.

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Answer

high; low

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Question

Arrhenius _____  dissociate in water and form H+ ions.

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Answer

acids

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Question

 Arrhenius ____ dissociate in water and form OH- ions.

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Answer

bases

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Question

 Bronsted-Lowry ______  are proton (H+) donors

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Answer

acids

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Question

Bronsted-Lowry bases are proton (H+) ________.

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Answer

acceptors

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Question

_________ is referred to as a measure of the concentration of hydrogen ions (H+) in a solution.

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Answer

pH

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Question

pOH is referred to as a measure of the concentration of ________ in a solution.

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Answer

hydroxide ions (OH-)

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Question

The smaller the pH, the more______  a solution is, and the higher the concentration of H+ ions in the solution.

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Answer

acidic

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Question

Acids have a pH _____ than 7 

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Answer

greater

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Question

Bases have a pH of _____ than 7 

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Answer

greater

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Question

True or false: the sum of the pH and pOH of an aqueous solution equals 14.

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Answer

True

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Question

The ______ the pOH value, the more basic a solution is, and the higher the hydroxide ion concentration.

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Answer

lower

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Question

What would be the pH of a solution containing a pOH of 9?

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Answer

5

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Question

True or false: strong acids and bases dissociate completely in solution.

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Answer

True

Show question

Question

Bronsted-Lowry acids are proton (H+) _____.

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Answer

donors

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Question

Bronsted-Lowry bases are proton (H+) _____ .

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Answer

acceptors

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Question

_____ is a measurement of the [H+] ion concentration in a solution.

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Answer

pH

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Question

Kis known as the ________.

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Answer

Acid dissociation constant

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Question

True or false: to calculate pH and pKa of weak acids, we need to know Ka.

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Answer

True

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Question

The higher the Ka of an acid, the _____ the acid will be. 

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Answer

stronger

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Question

True or false: pKa is referred to as the positive log of Ka.

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Answer

False

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Question

The higher the pK, the ______ the acid.

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Answer

weaker

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Question

Acids have a _____ pH value.

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Answer

low

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Question

Bases have a ______ pH value.

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Answer

high

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Question

Substances with a low pKa value are considered _______ acids.

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Answer

strong

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