Acetic acid ch 3 cooh is a weak acid with k a 18 x 10

20. Acetic acid, CH
3
COOH, is a weak acid with K
a
= 1.8 x 10
-5
. What amount of sodium acetate,
CH
3
COONa, must be dissolved in 250.0 mL of 0.30 M CH
3
COOH, in order to produce a buffer with
pH = 4.60?
(a) 0.12 mol
(b) 0.054 mol
(c) 0.022 mol
(d) 0.015 mol
(e) 0.078 mol
0.30 M HA K
a
= 1.8 x 10
-5
[A
-
] = ?
pH = 4.60; [H
+
] = 10
-4.60
= 2.51 x 10
-5
HA + H
2
O
←
→
H
3
O
+
+ A
-

22
HA +
H
2
O
←
→
H
3
O
+
+
Initial 0.30
Change
Equilibrium 2.51 x 10
-5
HA +
H
2
O
←
→
H
3
O
+
+
Initial 0.30 0
Change -X +X
Equilibrium 0.30 - X 2.51 x 10
-5
Y + X
X = 2.51 x 10
-5
HA +
H
2
O
←
→
H
3
O
+
+
Initial 0.30 0
Change -2.51 x 10
-5
+2.51 x 10
-5
+2.51 x 10
-5
Equilibrium 0.30-2.51 x 10
-5
2.51 x 10
-5
Y + 2.51 x 10
-5
([H
3
O
+
][A
-
])/[HA] = K
a
([2.51 x 10
-5
][Y + (2.51 x 10
-5
)])/[0.30 – (2.51 x 10
-5
)] = 1.8 x 10
-5
Due to small K
a
, assume that 2.51 x 10
-5
is very small and can be dropped.
Therefore, [HA] = 0.30; [A
-
] = Y
([2.51 x 10
-5
][Y])/[0.30] = 1.8 x 10
-5
(((([2.51 x 10
-5
] x [Y])/[0.30]) = (1.8 x 10
-5
)),X)
Y = 0.215mol/L = 0.0538 mol/0.250L
TITRATIONS and INDICATORS
A
-
Y
A
-
Y
+X
A
-
Y

23
21 Chem 162-2011 Final exam
Chapter 15b – Applic. of Acid & Base Equilibria (Buffers & Titrations)
Titrations and indicators
Which one of the indicators below would be best to use when titrating 0.1M morphine (K
b
=
1.6x10
-6
) with 0.3M HCl?
+
O
0
O
+
0
+X
X X