Question

Spectrophotometry

Reagents: Cobalt (II) chloride, Hexaaquacobalt (II) ion, Ferrocene, Crystal violet, Rose bengal, and Coumarin. How would you calculate/solve the following for each reagent?

1. Concentration of Solution = M

2. Absorbance of known conc. solution = A

3. Wavelength at maximum absorbance value = nm

4. Molar Absorption Coefficient, = M^-1 cm^-1

5. Absorbance of given solution = A

Answer 1

Since the question has a focus on Spectrophotometry, the answer can be predicted on the basis of Spectrophotogram (spectrum) obtained from the solution.

1) Concentration of Solution = M

It can be calculated as: M = n * V/M.W units are M i.e. molarity

where n = mass of sample taken

M.W. = molecular weight of sample

V = Volume of solution in litres.

Using different n and M.W., different molarities can be calculated.

2) Absorbance of known conc. solution = A

It can be calculated by Lambert Beer law. i.e. intensity of light while traveling through a medium is directly proportional to concentration and path length.

A = ε c. l.

Where A = absorbance (unitless)

ε = molar absorptivity constant with units M^-1 cm^-1

c = concentration in molars as found in part 1 above.

l = path length = 1cm (centimeter)

3) Wavelength at maximum absorbance value = nm

It can be found out from the spectrum only. There is no other way to actually do it mathematically.

4) Molar Absorption Coefficient, = M^-1 cm^-1

From Lambert Beer law:

A = ε c. l.

Where A = absorbance (unitless)

ε = molar absorptivity constant with units M^-1 cm^-1

c = concentration in molars as found in part 1 above.

l = path length = 1cm (centimeter)

Thus, ε = A/(c.l)

5) Absorbance of given solution = A

If the concentration is unknown and let us assume it to be 'X'.

Then, from above part 4, find out ε and use this value of to ε find Absorbance or concentration as per Lambert Beer Law.

i.e. A = ε c. l.

Thanks for posting the question. It was excellent.