Ph and POH

1. Acid/Base Chemistry
The pH scale and
what does it mean?
1Created by Muhammad Khawar Khan?

2. pH
•What is it?
•What does it mean?
•How do you measure it?
•Can you determine other things
from pH?
2Created by :M.khawar khan

3. pH an Overview

pH is the measure of the concentration of hydrogen
ions is a solution. pH is measured on a log scale.

pH = - log[H
+
]

pH is used to indicate the degree of acidity or
alkalinity (basic) of a solution.

A solution is of a water solvent (polar)
3Created by Muhammad Khawar Khan?

4. 
When there is a high [H
+
], the solution is acidic.

On the other hand when the [H
+
] concentration is
low, the solution is basic.

So if there is a high [H
+
], there must be a low [OH
-
]

If there is a low [H
+
], there must be a high [OH
-
]

So what does H
+
and OH
-
create?
H
2O
4Created by Muhammad Khawar Khan?

5. Water

Pure water should have a neutral pH, due to have
equal amounts of [H+] & [OH-].

Acid and bases are created by adding polar
molecules to water to increase [H+], or increase
[OH-]

The pH scale is used to measure the acidity or
alkalinity of a solution.
5Created by Muhammad Khawar Khan?

6. pH Scale
The pH scale ranges from 0 to 14. 0
being the most acidic and 14 being
the most basic.
So what does the pH tell us about
[H+] of a solution?
If something has a pH of:
pH 1 = [H+] = 0.1 M
pH 2 = [H+] = 0.01 M
pH 3 = [H+] = 0.001M
pH 4 = [H+] = 0.0001M
pH 5 = [H+] = 0.00001M
pH 6 = [H+] = 1X 10
-6
M
pH 7 = [H+] = 1X 10
-7
M
pH 8 = [H+] = 1X 10
-8
M
pH 9 = [H+] = 1X 10
-9
M
pH 10=[H+] = 1X 10
-10
M
pH 11=[H+] = 1X 10
-11
M
pH 12=[H+] = 1X 10
-12
M
pH 13=[H+] = 1X 10
-13
M
pH 14=[H+] = 1X 10
-14
M
Representative pH Values
Substance pH
battery acid 0.5
gastric acid 1.5-2.0
lemon juice 2.4
cola 2.5
vinegar 2.9
orange juice 3.5
acid rain 4.5-5.0
tea 5.5
milk 6.5
pure water 7.0
saliva 7.0-7.4
blood 7.3-7.5
sea water 8.0
soap 9.0-10.0
ammonia 11.5
household bleach 12.5
lye (sodium hydroxide) 13.5
6Created by Muhammad Khawar Khan?

7. How to Figure it out?
If you were given HCl with a concentration of 0.003M,
what would be the pH?
pH = -log [H+]
pH = -log[0.003M]
pH = 2.52
pH will not have a unit!!!
7Created by Muhammad Khawar Khan?

8. How does pH and pOH relate?
pH and pOH relate in the following way
pH + pOH = 14
pOH is the measure of the [OH
-
], in a solution.
What would be the pOH of an solution with a pH of
5.75?
pOH = 14 - 5.75
pOH = 8.25
8Created by Muhammad Khawar Khan?

9. If given pH
To determine the concentration from pH, one must do
the following:
[H+] = 10
(-pH)
So what is the [H+] of a solution with a pH of 12.05?
[H+] = 10
(-12.05)
[H+] = 8.91 X 10
-13
M
antilog
9Created by Muhammad Khawar Khan?

10. Sample Problem 1
What is the pH and pOH of an solution with [H+] of
3.33 X 10
-9
M?
1
st
 solve for pH pH = -log [H+]
pH = - log[3.33 X 10
-9
M]
pH = 8.48
2
nd
 solve for pOHpOH = 14 – pH
pOH = 14 - 8.48
pOH = 5.52 (acidic)
10Created by Muhammad Khawar Khan?

11. Sample Problem 2
What is the [H+] of a solution with a pOH of 4.14?
1
st
 determine pHpH = 14 – pOH
pH = 14 – 4.14
pH = 9.86 (basic)
2
nd
 determine [H+][H+] = 10
(-pH)
[H+] = 10
(-9.86)
[H+] = 1.38 X 10
-10
M
11Created by Muhammad Khawar Khan?

12. Sample Problem 3
What is the pH of a solution with a [OH-] = 2.35 X 10
-9
?
1
st
determine pOH pOH = -log [OH-]
pOH = -log[2.35X10
-9
M]
pOH = 8.63
2
nd
determine pH pH = 14 – pOH
pH = 14- 8.63
pH = 5.37 (acidic)
12Created by Muhammad Khawar Khan?

13. Sample Problem 4
What is the pH of a solution that was made from 4.57g
of HCl dissolved in 750 mL of water?
1
st
 determine moles of HCl
4.57g HCl
36.46 g HCl
1 mol HCl
=0.125 mol
HCl
13Created by Muhammad Khawar Khan?

14. Sample 4 ….
2
nd
determine concentration of [H+], since this is an
acid the [H+] would be the same as the concentration
of the whole molecule of HCl. We will get [H+] &
[Cl-], both with the total of 0.125 moles each.
So concentration will be moles / volume (L)
[H+] = 0.125 moles/ 0.750 L
[H+] = 0.333M
14Created by Muhammad Khawar Khan?

15. Problem 4…
Next determine pH
pH = -log [H+]
pH = -log [0.333M]
pH = 0.48 (very acidic)
15Created by Muhammad Khawar Khan?

16. Your Turn..
1.What is the [H+] of an acid with a pH of 5.56?
2.Determine the pH of a 0.0045 M HCl?
3.Determine pOH of a solution with a pH of 8.65?
4.What is the pH of a base with an [OH-] of
3.44X10
-5
M?
Ans: 1. 1.35
2. 2.75X 10
-6
M
3. 5.35
4. 9.54
16Created by Muhammad Khawar Khan?