Hf is a weak acid in water. write down the expression for the dissociation constant ka of hf.
Answers
Answer:
Hydrofluoric acid,
HF
, is a weak acid that does not ionize completely in aqueous solution to form hydronium cations,
H
3
O
+
, and fluoride anions,
F
−
.
More specifically, the acid is partially ionized in aqueous solution, the extent of ionization depending on the value of the acid dissociation constant,
K
a
.
The equilibrium that is established when hydrofluoric acid ionizes looks like this
HF
(
a
q
)
+
H
2
O
(
l
)
⇌
H
3
O
+
(
a
q
)
+
F
−
(
a
q
)
By definition, the acid dissociation constant for this equilibrium will be
∣
∣
∣
∣
∣
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
a
a
K
a
=
[
F
−
]
⋅
[
H
3
O
+
]
[
HF
]
a
a
∣
∣
∣
∣
−−−−−−−−−−−−−−−−−−−−−−−−
Keep in mind that the expression of the acid dissociation constant uses equilibrium concentrations.
Now, you know that the equilibrium concentration of hydrofluoric acid is equal to
0.025 M
.
Notice that every mole of hydrofluoric acid that dissociates produces one mole of hydronium cations and one mole of fluoride anions.
This means that if you take
x
to be the concentration of hydrofluoric acid that ionizes, you can say that this concentration will produce a concentration of
x
of hydronium cations and a concentration of
x
of fluoride anions.
This means that you have
[
F
−
]
=
x
and
[
H
3
O
+
]
=
x
Use the expression of the acid dissociation constant to find the value of
x
K
a
=
x
⋅
x
0.025
=
x
2
0.025
This will get you
x
=
√
0.025
⋅
K
a
x
=
√
0.025
⋅
6.8
⋅
10
−
4
=
4.12
⋅
10
−
3
Since
x
represents the equilibrium concentration of hydronium cations, you will have
[
H
3
O
+
]
=
4.12
⋅
10
−
3
M
The pH of the solution is defined as
∣
∣
∣
∣
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
a
a
pH
=
−
log
(
[
H
3
O
+
]
)
a
a
∣
∣
−−−−−−−−−−−−−−−−−−−−−−−−
In your case, you will end up with
pH
=
−
log
(
4.12
⋅
10
−
3
)
=
∣
∣
∣
∣
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
a
a
2.39
a
a
∣
∣
−−−−−−−−
Explanation:
Hydrofluoric acid, HF , is a weak acid that does not ionize completely in aqueous solution to form hydronium cations, H3O+ , and fluoride anions, F− .
pH=2.39 Explanation: Hydrofluoric acid, HF, is a weak acid that does not ionize completely in aqueous solution to form hydronium cations, H3O+,