1. A solution of thickness 2 cm transmits 40% incident light. Calculate the concentration of the solution, if the molar absorption coefficient is 6000 litre mol-¹ cm-¹.
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Answer:
The absorbance is given by:
A=log(I0/I)
I is the intensity of the light as measuredby the spectrometer.
I0 is set at 100 for a solution that absorbs no light at that particular wavelength.
In the first case
10% of the light is absorbed so 90% is transmitted and I = 90.
So in the second case I = 10 .
The Beer - Lambert Law gives us:
A=ε∘cl
ε∘ is the molar absorption coefficient.
c is the concentration in mol/l
l is the path length of the cell.
Putting in the numbers:
log (100÷90) = ε∘×10−3×1
log (100÷10) = 1 = ε∘ × c × 1
Dividing (1) by (2)⇒
log(1.11)=10−3÷c
∴ c = 10−3 ÷ 0.0457 = 0.0218 mol/l
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