The pH Scale & pH
Calculations
Chemistry 10
Mrs. Page
Learning Objectives:
• Explain and use the pH scale
• Use a pH meter and universal indicators to
determine the acidity or alkalinity of a
substance
• Solve problems involving pH and [H+]
Explain and use the pH scale
The pH Scale
• pH is a measure of how acidic or basic a
solution is, which means it is a measure
of the concentration of hydrogen ions
[H+] in a solution.
• The pH scale ranges from 0 to 14.
• Acidic solutions have pH values below 7
• A solution with a pH of 7 is neutral (pure
water)
• Basic/alkali solutions have pH values
above 7.
Explain and use the pH scale
pH Scale
• The pH scale is a way of
expressing the amount of
dissociation of acids and bases
in water.
• Instead of using very small
numbers, we just use the
NEGATIVE power of 10 on the
Molarity of the H+ (or OH-) ions.
Explain and use the pH scale
pH Scale
• The pH scale is a logarithmic scale (not linear).
• A change of 1 pH unit represents a tenfold change in the
acidity of the solution.
• For example, if one solution has a pH of 1 and a second
solution has a pH of 2, the first solution is not twice as
acidic as the second—it is ten times more acidic.
Measuring pH
Use a pH meter and universal indicators to determine
the acidity or alkalinity of a substance
• Indicators are substances that turn different colors in
the presence of an acid or base
•Universal Indicator – color
indicates pH
•Litmus paper – turns red
acid, turns blue base
Measuring pH
Use a pH meter and universal indicators to determine
the acidity or alkalinity of a substance
• Indicators are substances that turn different colors in
the presence of an acid or base
•Methyl
orange:
•Phenolphthalein:
commonly
used
in
commonly used in
titrations,
turns
orange
titrations, turns pink in
in
acidic
solutions
basic solutions
Measuring pH
• pH Sensor: digital tool
for measuring pH
• Must be used with a
GoPro and Logger Pro
software
• Gives reading as a pH
number
Use a pH meter and universal indicators to determine
the acidity or alkalinity of a substance
pH Calculations
Solve problems involving pH and [H+]
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Calculate the pH of a solution if the [H+] is 1 X 10-10
pH = -log [H+] = -log (1 x 10-10)
pH = 10
CAREFUL: on you calculator use (-) for negative & use
() around numbers
pH Calculations
Solve problems involving pH and [H+]
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Calculate the pH of a solution if the [H+] is 1.8 X 10-5
pH = -log [H+] = -log (1.8 x 10-5)
pH = 4.7
pH Calculations
Solve problems involving pH and [H+]
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Find the pH of a 0.15 M solution of Hydrochloric acid
pH = -log [H+] = -log (0.15)
pH = 0.82
pH Calculations
Solve problems involving pH and [H+]
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Find the pH of a 3.00 X 10-7 M solution of Nitric acid
pH = -log [H+] = -log (3.00 x 10-7)
pH = 6.52
Solve problems involving pH and [H+]
pH Calculations
If you have pH you can also calculate the [H+]
pH = - log [H+]
-pH = log [H+]
Take antilog (10x) of both sides and get
-pH
10
= [H+]
* to find antilog on your calculator, look for “Shift”
or “2nd function” and then the log button
Solve problems involving pH and [H+]
pH Calculations
If the pH of Coke is 3.12, what is the [H+] ?
-pH
10
= [H+]
-3.12
10
= [H+]
[H+] = 7.59 x
-4
10 M
Solve problems involving pH and [H+]
pH Calculations
A solution has a pH of 8.5. What is the Molarity of
hydrogen ions in the solution?
-pH
10
= [H+]
-13.4
10
= [H+]
[H+] = 3.2 x
-9
10 M
pH Calculations
Solve problems involving pH and [H+]
A solution has a pH of 1.5. What is the Molarity of
hydrogen ions in the solution?
-pH
10
= [H+]
-8.5
10
= [H+]
[H+] = 0.032M