Verification of Beer-Lambert Law Through a lab Experiment

Spectroscopy            

The branch of science which describes the interactions of electromagnetic radiation with matter is known as spectroscopy.

Principle of Spectroscopy

During the absorption some of the molecules falling in the path of the incident beam, collide with the photons of radiant energy. However only those photons get absorbed whose energy exactly equal to the difference of energy,  ΔE between the ground and the excited states of the molecules. ΔE = hv = hc/ λ

Spectrophotometer

“The instrument used to measure the amount of electromagnetic radiations absorbed by a compound is called spectrophotometer.” It consists of light source of radiation with the prism that can select the desired wavelengths which can pass through the sample of a compound being investigated. The radiation that is absorbed by the sample is detected and is recorded on a chart against the wavelength or wave number. Absorption peaks are plotted as minima in infrared, and usually as maxima in ultraviolet spectroscopy.

                                                      Fig; Working of Spectrophotometer

Beer-Lambert Law

The absorption of a light by a substance at a particular wavelength is proportional to molar concentration in a fixed path length. Mathematical it expressed as;

A = εcl

A is called absorption of a solution, c is the concentration of absorbing compound in moles /dm³, ε is called molar absorptivity, l is the length of the sample tube in cm.^[1]

Verification of Beer-Lambert Law Through a lab Experiment

Prepare the solutions of any salt of given concentration;

0.01M, 0.03M, 0.05M, 0.07M, 0.09M,

Then check absorption of solutions against  λ_max with help of spectrophotometer.

We examine the Cobalt(II) sulfate solution of given concentrations

Observations

λ_max of Cobalt(II) sulfate solutions = 512nm

Concentration	Absorption
0.01M	0.046
0.03M	0.145
0.05M	0.202
0.07M	0.381
0.09M	0.474

Graph

Graph; Showing the Abs Vs Conc.

According to Beer-Lambert Law

                                                                  ε = A/cl                                                              

If l = 0.1cm

Concentration	Absorption	ε = A/cl (mol-1cm-1)
0.01M	0.046	46
0.03M	0.145	48.3
0.05M	0.202	40.4
0.07M	0.381	54.4
0.09M	0.474	52.7

Discussion

According to Beer-Lambert Law molar absorptivity ε is the constant value for a substance. But our results deviate with Beer-Lambert Law.This is may be due to the presence impurities or some personal errors during solution preparations.

References

[1]. A Text Book of Organic Chemistry by Ghulam Rasool Chaudhary, First Addition 2009, Azeem Publishers Faisalabad

Preparation/Synthesis of 1,1'-Bi-2-naphthol

                                                    Synthesis of 1,1'-Bi-2-naphthol

Apparatus; Beakers,Round Bottom Flask,Laboratory Stand

Chemicals;2-Nephthol(C₁₀H₇OH),Ferric Chloride(FeCl₃)

Chemical Equation;

2 C₁₀H₇OH  + 2FeCl₃(aq)    ----------->   C₂₀H₁₄O₂   + 2FeCl₂ + 2HCl

Procedure

Dissolve 3.6g 2-Nephthol in 150 mL water and heat on water heater

Dissolve 7g FeCl₃ in 40 mL water

Add FeCl₃ Solution in hot 2-Nephthol solution

Oily drops of 2-Nephthol disappear and flaks of  1,1'-Bi-2-naphthol appeared

Heat further for 10 min

Filter hot suspension and wash with hot water

Dry the product and weigh

Theoratical Yeild

Mass of  2-Nephthol = 3.6g

Molar Mass of 2-Nephthol = 144.17gmol^-1

Moles of 2-Nephthol = 3.6g/144.17gmol^-1 = 0.02497 mol

Mass of FeCl₃ = 7g

Molar Mass of FeCl₃ = 270.3 gmol^-1

Moles of of FeCl₃ = 0.025

2 mol of 2-Nephthol produce = 1 mol of  1,1'-Bi-2-naphthol

1 mol of 2-Nephthol produce = ½ mol of  1,1'-Bi-2-naphthol

0.0249 mol of 2-Nephthol produce = ½ ×  0.0249 mol of  1,1'-Bi-2-naphthol

0.0249 mol of 2-Nephthol produce = 0.01249

2 mol of FeCl₃ produce = 1 mol of  1,1'-Bi-2-naphthol

1 mol of FeCl₃ produce = ½ mol of  1,1'-Bi-2-naphthol

0.025 mol of FeCl₃ produce = ½ × 0.025 mol of  1,1'-Bi-2-naphthol

                                             = 0.0125 mol of  1,1'-Bi-2-naphthol

2-Nephthol is a limiting reagent

Mass of 1,1'-Bi-2-naphthol = Moles × Molar Mass

                                            = 0.01249 mol × 286.32 gmol^-1

                                            = 3.58 g

Practical Yeild = 3.88 g

Disscusion

Experimently Obtained yeild is nearly greater than theoratical yeild .this happen due to the presence FeCl2 as biproduct.

The actual colour of crystalls of 1,1'-Bi-2-naphthol is white but we obtain greenishblack product.

This indicate presence of FeCl₂ with greenishblack colour in our product.