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General Chemistry
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NaCH
3
CO
2
+ CH
3
CO
2
H Buffer Solution
What mass of solid NaCH
3
CO
2
(molar mass =
82.0
g/mol) should be added to 1.00 L of
0.350 M
CH
3
CO
2
H to make a buffer of pH =
5.20
?
(K
a
for CH
3
CO
2
H =
1.8 × 10
-5
)
When working with buffers, we want to employ the Henderson-Hasselbalch Equation
pH
=
pK
a
+ log
[
A
]
/
[
HA
]
Info Provided
pH =
5.20
pK
a
=
4.74
[A] = base concentration=
?
[HA] = acid concentration =
0.350
setting up information to solve equation
we are given the K
a
,
so we can find the pK
a
a lowercase p simply means
'take the negative log of'
- log (
1.8 x 10
-5
) = -(-
4.74
) =
4.74
5.20
=
4.74
+ log
[
A
]
/
[
0.350
]
0.46 = log
[
A
]
/
[
0.350
]
10
0.46
=
[
A
]
/
[
0.350
]
2.884 =
[
A
]
/
[
0.350
]
[
A
] =
1.0094
So, the concentration of the base (NaCH
3
CO
2
) is
1.0094
moles
Now, we use the molecular weight provided to find grams
1.0094
moles
NaCH
3
CO
2
82.00
grams NaCH
3
CO
2
/
1 mole
NaCH
3
CO
2
=
82.77
g NaCH
3
CO
2
One should add 82.8 g of solid NaCH
3
CO
2
to prepare this buffer
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