## The pH Scale

#### The Basics

The pH of an acid or base indicates the concentration of H+ ions in solution.

Refer to the diagram below.

The pH scale goes from 0 to 14.

The chart below shows the concentration of hydrogen ions (H+) and the corresponding pH. (Brackets around a chemical, in this case H+, means “concentration of.”)

In order to understand the pH scale, you need to first understand numbers called exponents.

Exponents are the numbers that indicate how many times a number is multiplied by itself. For instance, 102 (we read "ten to the second power) is 10 X 10 or 100.

We also need to understand logarithms.

As an example, the logarithm of 102 is 2.

You also need to understand negative exponents.

For instance 10–2 is 0.01. To understand this, you take the imaginary decimal point for “1.” and you move it backwards (to the left) two places so that you get 0.01.

Just to make sure we understand this, which of these two number is bigger — 10–5^ or 10–7^ ? (Click on the arrow to see the answer.)
The number 10–5^ is bigger. 0.00005 is bigger than 0.0000001.

Remember earlier we learned that in a sample of water, a small number of water molecules dissociate (break apart)?

H2O —> H+ + OH-

How often does this happen? In a liter of water, it happens 10–7 moles per liter. (You really don’t need to know “moles/liter” means other than the fact that is a unit of concentration.

So in each liter of water, there would 10–7 moles/liter of H+ ions and 10–7 moles/liter of OH- ions. That’s why a pH of 7 is neutral.

Here are the facts you need to know about the pH scale:

• Acids have a pH less than 7.
• The lower the number (below 7), the stronger the acid (because the pH, the higher the concentration of H+).
• In acids, the [H+] > [OH-]* (see note below)
• Bases have a pH greater than 7.
• The higher the number (above 7), the stronger the base.
• In bases [H+] < [OH-]

`* Brackets [X] means "concentration of X."`

#### Relative strength of acids compared to each other

Suppose you had two acids, one with a pH of 3 and another one with a pH of 6. The acid with a pH of 3 is stronger but how much stronger? Here’s how to think about this. The strength of an acid can be indicated by the concentration of hydrogen ions (H+) in the acid. An acid that has a pH of 3 has 10–3 moles/liter of hydrogen ions. That’s the same as 0.001 moles/liter of H+ ions. An acid with a pH of 6 has 10–6 moles/liter of hydrogen ions. That 0.000001 moles/liter of H+ ions. So how much stronger is the pH 3 acid than the pH6 acid? The answer is 0.001 is a 1000 times greater than 0.000001. If you multiply 0.000001 by 1000, you would move the decimal point to the right 3 places (because there are 3 zeros in 1000), which would give you 0.001.

Here’s a video that might help you better understand acids, bases, and the pH scale.

Go back to the Session 1 Objectives