Chemistry Reference
In-Depth Information
11
Answers to problems
(a)
Ethanolamine is a base since it has an available lone pair
of electrons. The compound is a primary aliphatic amine,
and a primary alcohol. The p
K
a
value of 9.4 refers to
the ionisation of the conjugate acid of ethanolamine
(HOCH
2
CH
2
NH
3
). Since ethanolamine is a relatively
strong organic base, the conjugate acid is a weak acid and
therefore has a high p
K
a
of 9.4.
A1.1
(b)
Since ethanolamine is a base, aqueous solutions of
the
compound will be alkaline:
HOCH
2
CH
2
NH
2
3
OH
H
2
O
HOCH
2
CH
2
NH
3
The base abstracts a proton from water to generate OH
;
therefore the solution will be alkaline.
(c)
A 1% w/v solution contains 1 g in 100 mL
10 g in
1000 mL
10/61.08 mol in 1000 mL
0.164 mol L
-1
.
Using equation (1.3),
4
p
K
b
4
pH
p
K
w
-
log
c
where p
K
w
14, and
p
K
b
(14
9.4)
4.6
pH
11.3
which, as predicted, is alkaline.
A solution of ethanolamine (base, B) and its salt (BH
) will
function as a buffer solution. Since the required pH of 9.0 is
close to the p
K
a
value of 9.4, the buffer should be effective
over the required range.
(d)
HOCH
2
CH
2
NH
3
Cl
On addition of acid (H
),
HOCH
2
CH
2
NH
2
B
H
3
BH
