Lesson 14.4 The pH of a Solution
Suggested Reading

Zumdahl Chapter 14 Section 14.3
Essential Question

What is the pH scale?
Learning Objectives:.




Distinguish between aqueous solutions that acre acidic, neutral, or
alkaline using the pH scale.
Identify which aqueous solution is more acidic using the pH scale.
State that each change of one pH unit represents a 10-fold change in
the hydrogen ion concentration [H+].
Deduce changes in [H+] when the pH of a solution changes by more
than one pH unit.
Introduction
In the previous lesson you learned that whether an aqueous solution is acidic,
neutral, or basic depends on the hydrogen ion concentration. Because these
values are often very small, it is usually more convenient to describe acidity in
terms of pH, which is defined as the negative of the logarithm of the molar
hydrogen-ion concentration
pH = - log[H+]
For a solution in which the hydrogen ion concentration is 1.0 x 10-3 M, the pH
is
pH = - log[1.0 x 10-3] = 3.00
The rule for significant figures when working with logs is that the number of
decimal places after the decimal points in the pH equals the number of
significant figures reported in the hydrogen ion concentration.
The pH Scale
Expanding on the table from lesson 14.3 gives:
In acidic solution
In neutral solution
In basic (alkaline)
solution
[H+] > 1.0 x 10-7
[H+] = 1.0 x 10-7
[H+] < 1.0 x 10-7
pH < 7
pH = 7
pH > 7
This gives the pH scale
Great Explanation of the pH Scale:
Watch the following YouTube Video:
https://www.youtube.com/watch?v=pFK16GsU1e4
Weirder explanation:
Watch the following YouTube Video:
https://www.youtube.com/watch?v=oiwU0Zy0Gkc
Calculating pH from the Hydrogen Ion
Concentration
This is pretty straight-forward. Lets look at an example.
A sample of orange juice has a hydrogen ion concentration of 2.9 x 10-4 M.
What is the pH? Is the solution acidic?
Solution:
Plug the hydrogen ion concentration into the equation for pH given above, pH
= - log (2.9 x 10-4 ) = 3.54. The pH is less than 7.00, so the solution is
acidic. Note the use of significant figure; there are two significant figures in
the concentration, so the value for pH has two decimal places.
This video shows why each change of one pH unit represents a 10-fold
change in hydrogen ion concentration. This is one of the AP objectives, so
don't skip it if you are taking the AP exam!
Watch the following YouTube Video:
https://www.youtube.com/watch?v=Of7UBQT8FjM
Calculating pH from the Hydroxide Ion
Concentration
You can find the pH of a solution of known hydroxide ion concentration in one
of two ways:
1. Solve the ion product constant Kw = [H+][OH-] for [H+] and then use
pH = -log [H+], at 25 deg C.
2. Use pOH where pOH = -log [OH-] and pH + pOH = 14.00
For example, suppose you wish to find the pH of and ammonia solution whose
hydroxide ion concentration is 1.9 x 10-3 M. First, calculate pOH
pOH = -log [1.9 x 10-3 M] = 2.72
since pH + pOH = 14.00 we can solve for pH, where pH = 14.00 - 2.72 =
11.28.
YouTube Video
Interconverting pH and Hydrogen Ion Concentration
It is useful to be able to interconvert between pH and Hydrogen Ion
Concentration. Lets look at an example.
Calculating the Hydrogen Ion Concentration for the pH
The pH of human blood is 7.40. What is the hydrogen ion concentration?
Solution:
[H+] = antilog(-pH) = 10-pH
This gives
[H+] = antilog(-7.40) = 10-7.40 = 4.0 x 10-8 M.
Because pH was given to two decimal places we write the hydrogen ion
concentration with two significant figures.
HOMEWORK: Practice exercises 16.6 – 16.9