I = 0
Jonathon Nguyen
Biomolecular Dynamics
16323012
Experiment 2: Spectrophotometric Determination of Acidity Constant
“A Questions:
1)
What quantity is the spectrophotometric analysis measuring?
A spectrophotometer measures the intensity of light as a function of the wavelength of light. The
spectrophotometer measures the absorbance (A) by measuring the amount of light absorbed when the
light is shined through the sample and.
2)
Show how the value of the Ka for the mnitroaniline maybe derived from this quantity
A = log (I
O
÷ I)
NB: when Io = 0 it’s when the acid concentration at its highest concentration. This formula is derived
from the pKa formula:
pka = log [H
+
] [B]
[BH
+
]
3)
Define the quantities and units of “absorbance” and “molar absorption coefficient”.
“Absorbance” (A) is described as:
A
λ
=
log
10
(I ÷I
O
)
I: intensity of light at a particular wavelength (
λ
) that has transmitted light intensity.
IO: the initial intensity of light before it enters the sample also known as incident light
However, the molar absorption coefficient (
ε
) measures the strength of species to determine its
absorption of light at a given wavelength.
ε
is mainly measured in M
1
cm
1
or LMol
1
cm
1
.
4)
A solution containing 4.48ppm KMnO
4
has a transmittance of 0.309 in 1.00cm cell at 520nm.
Calculate the molar absorption coefficient of KMnO
4
.
At wt of KMnO4 = 158.036
158.036 × 100
1 
P a g e
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Biomolecular Dynamics
16323012
= 158036ppm
Therefore 1M of KMnO
4
is 158036ppm
If
1M
158036ppm
→
Then
4M
xppm
→
4.48ppm = 2.834797 × 10
5
M
Now
A = log
10
T =
ε
Cl
= log
10
(0.309)
= 0.51004152
Therefore
ε
Cl = 0.51004152
ε
=
0.51004152
.......
2.834797 × 10
5
M
Therefore
ε
= 1.80 × 10
4
is the molar coefficient of KMnO
4
.
Aim:
1.
To master the use of a spectrophotometric technique to measure the pKa of the weak acid m
nitroaniline(MNA).
2.
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 Spring '10
 meher
 pH, Sodium chloride, Biomolecular Dynamics, Jonathon Nguyen

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