Essay on Chemistry: Concentration and Path Length

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Practice Questions
[1] The absorbance is a dimensionless quantity, that is, a quantity without units. What will be the units of ∈, the specific absorptivity? Remember that 1 mL = 1 cm3.
From Beer-Lambert law:
A = ∈lC
A is the absorbance
∈ is the extinction coefficient l is the cell path length
C is the concentration
The units of ∈ are
∈ = [A] / [cm X C] → ∈ = [cm-1(C) -1] where C is the concentration units that you are using.
[2] How does the percent transmittance of a solution vary with (a) increasing concentration and (b) increasing path length? 2a
Percent Transmittance is inversely proportional to the concentration (more light goes through if the solution is very dilute). 2b
Percent Transmittance is inversely proportional to the Path length. Recall that A = -log[I/I0], thus the relation between the these quantities is logarithmic.
[3] The absorbance of an iron thiocyanate solution containing 0.00500 mg Fe/mL was reported as 0.4900 at 540 nm.
[a] Calculate the specific absorptivity, including units, of iron thyocyanate on the assumption that a 1.00 cm cuvette was used. [b] What will be the absorbance if (1) the solution is diluted to twice its original volume and (2) the solution is placed in a
5.00 cm cuvette?
3a
∈ = [A] / [cm X C]
∈ = [0.4900] / [1.00 cm X 0.00500 mg/mL] → ∈ = [98.0 cm-1(mg/mL) -1]
3.b1
A = ∈lC
A = [98.0 cm-1(mg/mL) -1] X 1.00 cm X [0.00250] → 0.2450
3.b2
A = ∈lC
A = [98.0 cm-1(mg/mL) -1] X 5.00 cm X [0.00250] → A = 1.225
[4] Reffer to problem 3.
[a] What percent of light is transmitted by the original iron thiocyanate solution?
[b] What concentration of iron thiocyanate will absorb 50% of the entering light?
4a
A =

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