Chemistry Reference
In-Depth Information
Some of these additional hydroxide ions may react with hydronium ions
to form neutral water molecules, as shown in this equation:
H
3
O
+
+ OH
-
H
2
O + H
2
O
The rest will go to increase the overall concentration of the hydroxide
ions in the solution. If we measure the pH of our new solution, we can find
the concentrations of hydronium and hydroxide ions, as well as another
measure called pOH. Let's suppose that the pH of our solution is mea-
sured as 13. These would correspond to a concentration of hydronium ions
of 1.0 × 10
-13
. So, if [H
3
O
+
] = 1.0 × 10
-13
, then [OH
-
] must be equal to 1.0 ×
10
-1
, as shown by the following equation.
k
w
[H
3
O
+
]
1.0 × 10
-14
1.0 × 10
-13
= 1.0 × 10
-1
[OH
-
] =
=
A concentration of hydroxide ions of 1.0 × 10
-1
corresponds to a pOH
value of 1. In case you haven't guessed, just as [H
3
O
+
] × [OH
-
] = 1.0 × 10
-14
,
pH + pOH = 14. If our solution has a pH of 13, then it must have a pOH
of 1. If it had a pH of 6, then it would have a pOH of 8.
Let's try a few examples that will test the ideas you learned in this lesson.
Example 1
What are the [H
3
O
+
] and [OH
-
] of a solution with a pH of 2?
We read this question as, “What are the concentration of hydronium
ions and the concentration of hydroxide ions of a solution with a pH of 2?”
A pH of 2 corresponds to a [H
3
O
+
] of 1.0 x 10
-2
. You can find the concen-
tration of hydroxide ions in a number of ways. One way is to make use of
the ion-product constant of water, k
w
, with the following equation:
k
w
[H
3
O
+
]
1.0 × 10
-14
1.0 × 10
-2
= 1.0 × 10
-12
[OH
-
] =
=
Alternatively, you could have reasoned that if the pH is 2, than the
pOH must be (14-2) 12, which corresponds to an [OH
-
] of 1.0 × 10
-12
.
Either way, you get the answers shown below:
Answer: [H
3
O
+
] = 1.0 × 10
-2
and [OH
-
] = 1.0 × 10
-12
The next example can be solved by anyone who remembers that pH +
pOH = 14.
