Unit: Acids, Bases,
and Solutions
Calculations with Acids and Bases
After today you will be able to…
• Explain the correlation to
strength of acids and bases to pH
and pOH scale
• Calculate pH, pOH, [H+], and
[OH-]
pH Scale
pH scale: the measure of acidity
of a solution
acidic
0
neutral
basic
7
+
pH=-log[H ]
[H+] = concentration in Molarity
14
Before we try an
example, you
will need to
locate the “log”
button on your
calculator.
Example:
What is the pH of a solution that
has an [H+]=1.5x10-4M?
-4
pH=-log[1.5x10 ]
pH=3.8
Example:
To do this calculation
+
What is the
[Hwill
] in need
a solution
you
to use
with pH=9.42?
the inverse log.
+
9.42=-log[H
] the “10x”
Locate
+] Usually it is
button.
-9.42=log[H
10-9.42the
=[H+]second function
of the -10
log
[H+]=3.80x10
M button.
pH Scale
pOH scale: the measure of
alkalinity (basic-ness) of a solution
basic
0
neutral
acidic
7
pOH=-log[OH ]
14
Example:
What is the pOH of a solution
that has an [OH-]=3.27x10-9M?
-9
pOH=-log[3.27x10 ]
pOH=8.49
Since the pH and pOH scales are
opposite each other:
pH + pOH = 14
Example: What is the pH in a
solution with a pOH=8.6?
pH + 8.6 = 14
pH=5.4
Summary…
+
pH=-log[H ]
pOH=-log[OH ]
pH + pOH = 14
Ion-Product Constant for Water
Water will self-ionize to a certain
extent into its individual ions.
Because of this, the following
relationship can be used:
+
-14
[H ][OH ]=1.0x10 M
Kw
Example:
+
[H ]
What is the
in a solution
with [OH-] = 6.73x10-5M?
+
-5
-14
[H ][6.73x10 ]=1.0x10
[H+] = 1.49x10-10M
Sometimes multiple
formulas must be
used to carry out
these calculations:
Example:
What is the pOH in a solution with an [H+] = 2.17x10-5M?
(Note: there are multiple ways to do this problem!)
[H+][OH-]=1.0x10-14
[2.17x10-5][OH-]=1.0x10-14
[OH-]=4.61x10-10M
+
pH=-log[H ]
pOH=-log[OH ]
pH + pOH = 14
+
-14
[H ][OHpOH=9.34
]=1.0x10
pOH=-log[OH-]
pOH=-log[4.61x10-10]
Example:
What is the [OH-] in a solution with pH=8.1?
(Note: there are multiple ways to do this problem!)
pH + pOH = 14
8.1 + pOH = 14
pOH = 5.9
+
pH=-log[H ]
pOH=-log[OH ]
pH + pOH = 14
+
-14
[H ][OH
]=1.0x10
[OH ]=1.3x10 M
pOH=-log[OH-]
5.9=-log[OH-]
-
-6
Questions?
Begin WS4