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At 25°C in pure water, the concentration of hydronium ions is extremely small,
-7
1 x 10
Moles/liter, and balanced by an equal concentration of hydroxyl ions. The
equilibrium constant, Kw of water is the product of the hydronium ion and hydroxyl
ion concentrations:
+
–
-
Kw = [H
O
] [OH
] = [1x10
3
Since the hydronium ion concentration is 1 x 10
the neutral pH, as stated above:
-7
pH = -log [1 x 10
] = 7
In aqueous solutions at 25°C, the product [H
constant. Therefore, an increase in concentration of either term, [H
result in a decrease in the other. For example, if a strong base, sodium hydroxide (NaOH)
is added to water to the extent that its final concentration is 0.01 Moles/liter, the concen-
–
tration of the OH
ion is 0.01 Moles/liter, and the concentration of the H
decrease according to the Kw equation as follows:
+
–
[H
O
] = Kw/ [OH
] = 1 x 10
3
The pH of this solution is:
-12
pH = -log [1 x 10
] = 12
This high pH indicates that the 0.01 M NaOH solution is strongly alkaline; the concentra-
tion of hydronium ions is extremely small.
Figure 1
0
1
2
3
4
5
more acidic
-7
-14
7] [1x10
] = 1x10
-7
Moles/liter, the pH of pure water is 7 ,
+
–
O
] [OH
] or the Kw of water must remain
3
-14
-2
-12
/ 1 x 10
= 1 x 10
Neutral
6
7
8
9
10
more basic
pH
pH THEORY
+
–
O
] or [OH
], will
3
+
O
must
3
11
12
13
14
Appendix
61
- Quick Links:
- Ph Electrodes
- Standardizing
- Ph Measurement
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