The pH Scale, Solutions & Buffers

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

What do lemons, drain cleaner and blood have in common? They are either acids or bases. This lesson will explore acids and bases, giving examples and definitions. It will also explain how to calculate the pH and the pOH, and will define buffers.

Definition and Examples of Acids and Bases

Right now I bet you can find at least five acids and five bases in your home. What in the world are acids and bases, you ask? Excellent question. An acid is a substance that has the following characteristics:

  • A sour taste
  • A pH less than 7 on a pH scale
  • And a substance that will donate hydrogen ions, or positively charged hydrogen atoms, when it's dissolved in water.

Before we define bases, let's talk about the pH scale, which measures if a substance is an acid, a base or a neutral substance (neither an acid nor a base). As mentioned moments ago, a pH of less than 7 indicates an acid. The closer a substance is to zero, the more acidic it is.

pH scale with examples
pH scale

A base, which is sometimes referred to an alkaline substance, has the following characteristics:

  • A bitter taste
  • A pH greater than 7 on the pH scale
  • And a substance that will donate hydroxide ions when it's dissolved in water. A hydroxide ion is oxygen attached to a hydrogen atom. Hydroxide ions have a negative charge. Bases will also accept hydrogen ions from acids.

The closer a substance is to 14 on the pH scale, the more basic it is.

So, what are the acids and bases you can find in your home? Take a look in your kitchen. If you find lemons, vinegar, tomatoes, cherries and grapefruit, you have found five acids! Now take a look at your cleaning supplies. If you have baking soda, drain cleaner, dishwasher detergent, cleaners containing ammonia, and oven cleaner, you have found five bases! Take a look at the table to see other common acids and bases you may be familiar with.

Acid Examples Base Examples
Hydrochloric acid Sodium hydroxide
Colas Antacid
Stomach acid Blood

Calculating pH and pOH

Now that you have an understanding of what acids and bases are, let's take a moment to learn how to use a mathematical formula to calculate the pH and pOH. You know that the pH is a scale, but pH actually stands for the power of hydrogen because it is a measurement of how many hydrogen ions a substance has. Remember, acids release hydrogen ions, so if a substance has a lot of hydrogen ions, chances are it's an acid. Oh yeah, and pOH stands for the power of the hydroxide ion (remember, bases release hydroxide ions when they are dissolved in water). Take a moment to read through the steps you need to calculate the pH.

First off, you need to know the molarity (represented by an M) of hydrogen ions, which means the number of moles per liter of the solute. So many mysterious words there -- let's unpack them. In chemistry, a mole isn't a furry critter that lives underground; instead it's just a way to express a number. For example, a dozen is 12, a couple is two and a mole is 6.02 x 10^23. A solute is the substance that is dissolved in water.

Now that you know a little about molarity, you are going to use that number in a formula: pH = -log (H3O+), where H3O+ is the molarity.

Let's do an example. Find the pH of a substance with a 0.0035 M. Since you know the molarity, go ahead and plug it into the formula:

pH = -log(0.0035) Now, plug this into the calculator. You should get your pH = 2.46.

Now for the pOH.

Again, you need to know the molarity. This time, however, you need to know the molarity of hydroxide ions. You will use a similar formula, but (again) using the molarity of hydroxide ions, not hydrogen ions. Here's the formula: pOH=-log(OH-)

Let's do an example. What's the pOH of a substance that has a molarity of 0.0085? Use the formula:

pOH = -log(0.0085), which gets you: 2.07.

Before we move on, it's worth noting that the pH + pOH of a substance will equal 14. So in our first example, if our pH is 2.55, that means the pOH of that substance is 11.45.

Weak Acid/Bases and Buffers

When acids and bases are strong, it just means they completely ionize in water. For example, if you put the strong base, sodium hydroxide (NaOH) in water, it becomes Na+ and OH-. Ionization just means the molecule breaks apart into ions (in this case Na+ and OH-).

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