Lab 2: pH and Buffers

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Lab 2 pH and Buffers
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Bronsted-Lowry Acids and Bases
Acid-base reactions involve the transfer of
ions from one substance to another
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Acids and bases can be defined in terms of their
ability to transfer protons
An acid is a substance (molecule or ion) that can
transfer a proton to another substance
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A base is a substance that can accept a proton
from another substance.
The emphasis here is on the transfer of protons
For our purposes, these transfers occur in
aqueous solution
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Example A generic reaction of an acid with water
HX is any substance that acts as an acid (proton
donor)
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Autoionization of Water
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One of the most important chemical properties of
water is its ability to act as either and acid or
a base, depending on the circumstances
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In the presence of an acid, water acts as a
proton acceptor in the presence of a base, water
acts as a proton donor
It is not surprising then that one water molecule
can donate a proton to another water molecule
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..
..
..

H
O

H
O
H
O
H

H
H
H
..
.
.
O
H
..
Autoionization of water One water molecule can
donate a proton to another water molecule
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At room temperature, only about one out of every
water molecules is ionized at any given time
As a result, pure water consists almost entirely
of H2O molecules and is a very poor conductor of
electricity
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However, autoionization of water is still very
important. Because it is an equilibrium process,
we can write an equilibrium-constant expression
for it
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This is called the ion-product constant for
water No matter what the concentration of either
ion is, their product must always equal
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A solution for which
is said to be neutral
In most solutions, these ions are not equal in
concentration. As the concentration of one
increases the other must decrease so that their
product equals
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Example Calculate the values of
in a neutral solution at 25C
By definition, in a neutral solution
Since the concentrations are equal, we can assign
the variable x to each ion
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We get
In an acid solution,
is greater
than
In a neutral solution
it is less
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Example Calculate
in a solution in which
is 0.010M
We start by using this equation
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This sets the stage for approaching concepts of
pH First, we need to review exponential and
logarithmic functions. Understanding how these
functions work is important in understanding the
pH scale
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Exponential Functions
Inverse
y is the exponent to which b must be raised to
yield x
We need a more compact system of notation to
express the inverse
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Definition
We define the expression
to mean the exponent to which b must be raised
to yield x
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We read this expression as log base b of x
This is an example of a logarithmic function
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It is important to note now that
and
are equivalent expressions
In these expressions, the variable b is a
constant. For our purposes, it is the number 10
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What this means is that we are using a base-ten
system Log base 10 is called the common
logarithm, and corresponds to the log key on a
graphing calculator
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This becomes the exponent to the base
Then the base raised to the exponent is set equal
to x
This is the base
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If we replace b in the functions with the number
10, we get
and
We can use these expressions to solve problems in
pH
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The concentration of
In an aqueous solution is usually quite small. We
therefore express the concentration of hydrogen
ions as the negative logarithm of
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pH is given as
We can apply these expressions when solving pH
problems
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For instance, we calculated the hydrogen ion
concentration in a slide above What is the pH of
that solution?
We determined that the hydrogen ion concentration
is
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If pH is given as
then the pH of our solution is
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Example A sample of freshly pressed apple juice
has a pH of 3.76. Calculate
We can rewrite this using the known values
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or
To solve for H, we can rewrite this expression
in the form
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becomes
The variable x corresponds to the hydrogen ion
concentration
The value is
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