Can you identify the following as weak acid, strong acid, weak base, strong base, or neutral? How do you determine this?
Answers
Simply from the
K
a
, or the
pKa
. Or, the
K
b
, or the
pKb
, if comparing by dissociation in water.
The higher the
K
a
or lower the
pKa
, the stronger the acid.
The lower the
K
a
or higher the
pKa
, the weaker the acid.
Converting from
K
a
to
K
b
,
K
b
=
10
−
14
K
a
.
Converting from
pKa
to
pKb
,
pKb
=
14
−
pKa
.
Weaker acids have stronger conjugate bases and vice versa, so there's no need to think about it much more than that.
These are all relative to water, so clearly, pure water is considered neutral.
KOH
, an alkali metal hydroxide, easily dissociates to release
OH
−
into solution, which classifies it as a strong base. No need to check
pKa
here.
HCl
, a binary acid, easily dissociates to release
H
+
into solution, with
pKa
≈
−
7
. Strong acid.
CH
3
COOH
is a carboxylic acid, whose
pKa
is about
5
(actually, this one is
4.74
). Often called acetic acid. Considered to be a weak acid, but still dissociates its proton significantly.
H
2
SO
4
has a
pKa
1
of
−
3
so it dissociates very easily. Strong acid.
H
2
CO
3
has a
pKa
1
of about
6.4
. Considered to be a weak acid, but still dissociates its first proton significantly.
H
3
PO
4
has a
pKa
1
of about
2.16
. Considered to be a relatively weak acid, but still dissociates its first proton significantly.
H
2
O
has a
pKa
of about
15
. Obviously neutral, since it cannot be a solute in its own solution and alter its own pH.
H
3
O
+
has a
pKa
of about
−
1.7
, so it's a strong acid.
HCN
has a
pKa
of about
9.2
. Considered to be a weak acid, but still dissociates its first proton significantly.
NH
4
OH
forms like this:
NH
3
(
a
q
)
+
H
2
O
(
l
)
⇌
NH
+
4
(
a
q
)
+
OH
−
(
a
q
)
K
b
=
[
NH
+
4
]
[
OH
−
]
[
NH
3
]
=
1.8
×
10
−
5
⇒
pKa
≈
9.25
.
Thus,
NH
4
OH
dissociates significantly to form
NH
3
in solution, and most consider it a strong acid, even though its
pKa
is so close to that of
HCN
.
Lastly, make sure you know that acidity and basicity is all relative. You MUST compare the
p
K
a
s. Strong acids are only considered strong because they dissociate significantly in water.
If water had a lower
pKa
, these strong acids would be weaker. Similarly, if you dissolved
HCl
in
H
2
SO
4
,
HCl
would be considered a weak acid
I hope it will help you