pH and pOH
Self-Ionization of Water
We can tell how acidic or basic a solution is by looking at the concentrations
of H+ and OH-. A neutral solution will have equal concentrations of H+ and
OH-. An acidic solution will have a greater concentration of H+ than OH-,
but all aqueous solutions have H+ and OH- ions in them, just in different
concentrations. All aqueous solutions have both ions because a small
number of water molecules ionize to form H+ and OH-. The equation below
shows how a proton is taken from one water molecule and placed on another
water molecule.
H2O + H2O <----------------------------->
+
-
H3O + OH
The ionization produces hydronium ions and hydroxide ions. An
abbreviated form of the equation is shown below.
H2O <------------------------------>
+
-
H + OH
In a neutral solution, the concentrations of H+ and OH- will be the same, and
will be equal to 1 x 10-14. This number is called the ionization constant for
water (Kw). In an aqueous solution, the [H+] x [OH-] will equal this
number. If a solution is acidic the [H+] will increase and the [OH-]
concentration will decrease, but the two multiplied times each other will still
equal Kw (1 x 10-14). This is possible because as acid (H+) is added to the
solution the equilibrium shifts to the left and decreases the OHconcentration. In a basic solution the [OH-] will increase and the
equilibrium will shift to the left again, only this time it will decrease the
[H+]. The expression below shows the relationship between Kw and the H+
and OH- concentrations.
Kw = 1 x 10-14 = [H+] x [OH-]
Using this relationship, if you know one of the concentrations in an aqueous
solution, you can calculate the other concentration.
Sample Problem
An aqueous solution contains a hydroxide (OH-) concentration of 3.8 x 108
M. What is the H+ concentration in this solution?
Solution
Kw = 1 x 10-14 = [H+] x [OH-]
1 x 10-14 = [H+] x [3.8 x 10-8]
1 x 10-14
------------- = [H+] = 2.6 x 10-7
We can tell that this solution is
+
acidic because the [H ] is greater than
3.8 x 10-8
the [OH-] concentration.
pH
The pH scale is a measure of the acidity of a solution. The pH scale runs
from 0-14, with 0 being very acidic and 14 being very basic. A neutral
solution will have a pH of 7. pH values are calculated using the [H+]
concentration of a solution. The formula for calculating pH is shown below.
pH = -log [H+]
Sample Problem
What is the pH of a solution with an [H+] concentration of 4.9 x 10-5M? Is
this solution basic or acidic?
Solution
The pH can be calculated by using the formula above.
10-5]
pH = -log [4.9 x
pH = - (-4.3)
pH = 4.3
The solution is acidic because the pH is below 7.
pOH
The pOH scale is a measure of how basic a solution is. The pOH scale runs
from 0-14, with 0 being very basic and 14 being very acidic. A neutral
solution will have a pOH of 7. The pOH scale is the opposite of the pH
scale, and either one can be used to determine the acidity of a solution. The
pOH scale is used more often when the hydroxide ion (OH-) is known rather
than the H+ concentration. The formula for calculating the pOH of a
solution is shown below.
pOH = -log [OH-]
Sample Problem
What is the pOH of a solution with an [OH-] concentration of 8.8 x 10-9M?
Is this solution acidic or basic?
Solution
The pOH can be calculated using the above formula.
]
pOH = -log [OHpOH = -log [8.8 x
-9
10 ]
pOH = - (-8.1)
pOH = 8.1
The solution is acidic because the pOH is above 7.
If the pH of a solution is known, the pOH can be calculated using the
relationship shown below.
pH + pOH = 14
For the example above, the pOH was 8.1. The pH can be found by
substracting the pOH from 14.
pH + 8.1 = 14
pH = 14 - 8.1
pH = 5.9