Section 4.5
pH and
its measurement
Introduction
pH is a common way to express
acidity and basicity
Definition :
pH is the Unit of Measure used
to express the degree of acidity
of a substance.
pH = - log [H3O+]
In pure water
[H3O+] = [OH-] =1 x 10 -7M
Therefore the pH of pure water is :
pH = - log [H3O+] = - log (1 x 10 -7)
pH = 7
The pH scale
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
[H3O+] > [OH-]
acidic
[H3O+]
[H3O+]
= [OH ]
< [OH ]
basic
neutral
It is used to show the acidity of a soln.
Q1. What is the pH of HCl solution
with [H3O+] of 1 x 10 -2 M ?
pH = - log
+
[H3O ]
= - log (1 x 10
pH = 2
-2)
Q2. If the pH of Diet Coke is 3.12,
+
what is the [H3O ] ?
3.12
pH = - log [H3O+]
log [H3O+] = - pH
Take antilog and get
[H3O+] = 10-pH
[H3O+] = 10-3.12
[H3O+] = 7.6 x 10-4 M
Q3. What is the pH of the 0.001M
NaOH solution?
 
1.00 x10 14  H  0.001
H 
H 

pH = - log [H3O+]
= - log (1 x 10

1.00 x10 14

0.001
 1.00 x10 11 M
-11)
pH = 11
Is this solution acidic or basic?
Measurement of pH
Methods
pH1 2
pH8 9
7.00
pH paper
pH meter
Experiment on measuring pH
Material
1
2
3
4
5
6
lemon juice
vinegar
soap
baking soda
aspirin
household ammonia
pH Acidic/Alkaline
Test the pH of most common
solution - pH panel
http://www.miamisci.org/ph/phpanel.html