pH Scale

Definition of pH

First of all, we need to define pH.

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

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:

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.

HCl 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.

KOH 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:

$\mathrm{HX}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{X}}^{-}\left(\mathrm{aq}\right)$

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

$\mathrm{BOH}\left(\mathrm{aq}\right)\to {\mathrm{B}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

pH Scale: Range

The pH scale looks like this:

pH 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.

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!

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:

$=-{\log }_{10}\left[{\mathrm{H}}^{+}\right]$

Let's look at an example!

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

$\left[{\mathrm{H}}^{+}\right]={10}^{-\mathrm{pH}}$

Let's solve another example!

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 of commons substances, StudySmarter Originals.

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

_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.}