Lesson 14.5 Acid-Base Equilibria
Suggested Reading

Zumdahl Chapter 14 Section 14.4 & 14.5
Essential Question

How is the pH of acid and base solutions calculated?
Learning Objectives:.



Calculate the pH of a strong acid or base solution.
Solve equilibrium problems involving acidic and basic solutions.
Calculate the pH of a weak acid or base solution.
Introduction
Lets jump right in.
Calculate the pH of a Strong Acid or Base Solution
Recall from the previous lesson that pH is a measure of the concentration of
hydrogen ions in a solution. Strong acids like hydrochloric acid that you
normally use in the lab have a pH around 0 to 1. The lower the pH, the higher
the concentration of hydrogen ions in the solution. The equation for pH is
given by
Note: If you are asked to define pH on an exam, simply write down the
expression in black. Never try to define it in words - it is a waste of time, and
you are too likely to leave something out (like mentioning that the
concentration has to be in mol dm-3). In the expression, above, the square
brackets imply that, so you don't need to mention it. This is also true for other
expression like the law of mass action, ideal gas law, etc.
Suppose you had to determine the pH of a 0.1 mol dm-3 hydrochloric acid. All
you have to do is work out the concentration of the hydrogen ions in the
solution, and then calculate the pH. With strong acids this is easy.
Hydrochloric acid is a strong acid - virtually 100% ionised. Each mole of HCl
reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride
ions That means that if the concentration of the acid is 0.1 mol dm-3, then the
concentration of hydrogen ions is also 0.1 mol dm-3. Now use your calculator
calculate the pH.
pH = - log10 [0.1] = 1
The pH of this acid is 1.
The pOH for a strong base is calculated in exactly the same way, except that
you use the concentration of hydroxide ion.
Calculate the pH of a Weak Acid
In order to calculate the pH of a weak acids and bases, you first must
determine the equilibrium concentration of the hydrogen or hydroxide ion. This
involves setting up an ICE table just like you did in Chapter 14. The handout
will walk you through it, but you should be familiar with the process.
Read the following handout on calculating the pH of a weak acid at this web
site:
http://science.csustan.edu/stone/2090/2090ICE.pdf
Calculate the pH of a Weak Base
Weak Bases follow the general equation:
Weak Base + H2O ⇌ conjugate acid + OH-
Example: NH3 + H2O ⇌ NH4+ + OHThe equilibrium expression is given by
Kb = [conjugate acid][OH-]
[weak base]
Example: Kb = [NH4+][OH-]
[NH3]
To calculate the pH of a weak base, we must follow ICE process, however,
our solution will allow us to determine pOH. We must not forget to use the
pOH to calculate the pH. Watch out for this!
Example: Determine the pH of 0.15 M ammonia with a Kb=1.8x10-5
Answer:
Write the equilibrium equation for the base:
NH3 + H2O ⇌ NH4+ + OHWrite the equilibrium expression with the Kb value:
Kb = [NH4+][OH-] = 1.8x10-5
[NH3]
Ice it!:
NH3 + H2O ⇌ NH4+ + OH
-
Ice:
0.15 M
Change:
-x
Equilibrium: 0.15 - x
----
0M
+x
x
0M
+x
x
Substitute the variables (disregard the "-x" because it is so small compared
to the 0.15)
and solve for [OH-]:
Kb = 1.8x10-5 = (x)(x) = x2
(.15 - x) .15
x = [OH ] = 1.6x10-3 M
pOH = -log[OH-] = 2.80
pH = 14.00 - 2.80 - 11.20
-
A dry but straightforward example of the problem type outlined above.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=T679W7VjFAw
HOMEWORK: PRACTICE EXERCISES 16.10-16.12, 16.15, 16.17
Book questions: (final review over everything from all ch. 14 lessons)
Page 672 questions 5, 19, 25, 27, 29, 31, 32, 33, 37, 38, 41, 42, 43, 47, 59, 61, 99