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Contoh 1. Calculating from pH [H 3O ] or [OH ]
+
-
Calculate the pH of each of the following solutions at 25 C and indicate whether the
0
solution is acidic or basic.
(a) [H 3O] = 1.8 X 10 M (b) [OH ] = 1.3 x 10
-2
-4
-

Solution
(a) To calculate pH, substitute the given pH = -log[H 3O ]
+
[H 3O ] into the pH equation. Since pH = -log(1.8 x 10-4 )
+
<7, this solution is acidic. = (-3.74)
= 3.74 (acidic)
-
(b) First use K w to find [H 3O] from [OH ] [H 3O ] = K w = 1.0 x 10 -14
+
[H 3O ] (1.3 x 10 ) = 1.0 x 10-14
+
-2
Then substitute [H 3O] into the pH [H 3O ] = 1.0 x 10 = 7.7 x 10 -13 M
+
14
expression to find pH. 1.3 x 10
-2
+
pH = - log [H 3O ]
Since pH >7, this solution is basic. = - log (7.7 x 10 )
-13
= - (-12.11)
= 12.11 (basic)

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