Measuring the Strength of
Acids - pH I love pH and
 Questions to answer today:




science and
math!
What scale do we use?
What does this scale measure?
What does the “strength” of the acid tell us?
How to calculate pH?
 But first, we start with earthquakes...
 And how they are related to pH...
Wednesday, April 27, 2011
Let’s Get Ready to Rumble!!!
 Earthquakes are really powerful.
 We need a scale to measure the enormous amounts of
energy released
 Since some earthquakes are small and some are huge, we
can not use a “normal” scale.
 Scientists decided to use Logarithms due to ease and
convenience to measure earthquakes
 And because logs are awesome!
 Let’s see what this means...
Wednesday, April 27, 2011
Richter Scale - A Logarithmic Scale
A 5 is ten times
more energy
than a 4
Every number
on a log scale
represents a
factor of ten
7
6
5
.
4
Wednesday, April 27, 2011
3
2
1
The pH Scale
pH scale : measures acidity
Søren Sorensen
(1868 - 1939)
ACID
BASE
10x10x
100x 10x
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
NEUTRAL
Each step on pH scale represents a factor of 10.
pH 5 vs. pH 6
pH 3 vs. pH 5
pH 8 vs. pH 13
Wednesday, April 27, 2011
(10X more acidic)
(100X different)
(100,000X different)
Acid
Base
pH = 7
[H+]
Acidic
pH = 0
Basic
Neutral
[H+] = [OH-]
Pictures are
great, but I want
some math!
Yeah, you did say
logarithms, right?
Wednesday, April 27, 2011
[OH-]
pH = 14
The Math Part! Yay!
 Use this formula: pH = -log[H+]
 The log button is on your calculator. Find it, become
friends.
 The [H+] is a fancy way of writing “the molarity of
hydrogen ion.”
 In other words, just use the molarity of your acid.
 Example problem!?
 You mix 0.024 moles of HCl in 500mL of water.
What is the pH of the solution?
M = mol / L
pH = -log[H+]
Wednesday, April 27, 2011
M = 0.024 / 0.500 = 0.048M acid
pH = -log[0.048] = 1.3
A Few Things Before We Depart ...
 pH does not have any units. It’s just a number.
 For now we are just focusing on the acid end. We
will get to the base end (pOH) next week.
 The pH scale is not linear; it is logarithmic, which
means that a pH of 3 and 6 is 1000x different.
 0 is the strongest acids, 14 is the strongest
bases.
 Remember that the formula for pH just requires
the molarity of your acid and the log button!
Easy!
Wednesday, April 27, 2011