# Lab report Relating the concentration of light absorbing substance to

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Labreport: Relating the concentration of light absorbing substance tothe light absorbance quantitatively

Aligand is made up of a central metal ion that is surrounded bymolecules to form a complex substance. Ligands form coordinatecovalent bonds by donating lone pairs of electrons to d-orbitals. Theenergy differences existing between the d-d-orbitals increases thestrength of ligand field. The wavelength with energy that is equalto the energy difference is usually absorbed and the wavelengths thatare transmitted in the form of the visible light are observed as itscolor (PV Education Organization 1).

Opticaldensity, on the other hand, is the measure of the relativeconcentration of the ion that is formed in the solution. The initialoptical density is usually measured at the wavelength where maximumabsorption of light occurs. Optical absorbance that s measured atthis wavelength indicates maximum value of the solution. Metal ionand ligands are the key limiting factors.

TheBear-Lambert Law is concept that holds that the absorbance of a givenmaterial sample should be directly proportional to the thickness ofthat material (Dartmouth College 1). The combination of Beer’s lawand Lambert’s Law is expressed as follows

Absorbance= abc

=-logT

=logPoP

Where

A= absorptivity of a liquid

B= optical path length

C= concentration of absorbing material

Theintensity decreases as the light traverses through the medium thatcontains the analyte. The length of the medium through which thelight passes is directly proportional to the number of absorbersalong the path and the level of attenuation.

Transmittanceof a solution refers to the fraction of an incident radiation that istransmitted by a given solution.

TransmittanceT = PPo

A= -logT = log PoP

Bear’slaw dictates that absorbance should be directly proportional to levelof concentration of species c and path. Therefore

A= -logPoP = abc

Theconcentration can be expressed in b centimeters and moles per liter

A= bc

Apparatus

• 50mL and 10mL volumetric flasks

• 5mL and 2mL pipettes

• 500mL and 100mL beakers

• Cuvette

• UV/Vis spectrometer

Materials

• Benzene

• Cyclohexane

• Unknown solution U1.2

• 0.1M KSCN

• Acetone

• 3M H2SO4

• 10-3M Fe2 (SO2)3 in 1M H2SO4

Experimental

PartA: The determination of the concentration of benzene found incyclohexane

Astock solution was formed by increasing 1 cm3 of benzene t0 50 cm3with cyclohexane. This solution was labeled as X. About 1 cm3 ofsolution X was made up to 50 cm3 by adding cyclohexane and labeledsolution as Y. Solution Y was then used in the preparation ofsolutions 1-5. Absorbance of all solutions was recorded where thereference range was 230-270 nm.

Table1: The determination of the concentration of benzene found incyclohexane

 Solution Volume of Stock Solution (Y) used /mL Volume of Cyclohexane used /mL Total Volume of Solution / mL 1 8 2 10 2 6 4 10 3 4 6 10 4 2 8 10 5 1 9 10

PartB: Determining the effect of the formation of the complex

About10mL (Fe)2(SO4)3, 100mL KSCN, and 100mL distilled water were mixedand stirred in beaker (500 mL size) using a glass rod. Absorbance wasthen measured at the start of mixing. The solution was then exposedto the sunlight. Absorbance was again measured at intervals of 5, 10,15 &amp 20 minutes. The 100mL distilled water was then substitutedwith a similar volume of acetone and the same process repeated.

Results

PartA

Table2: Readings for the absorbance of benzyne when put in cyclohexane atthe maximum wavelength of 261nm.

 Solution Absorbance at 260 nm 1 0.260 2 0.222 3 0.180 4 0.156 5 0.061 Unknown U1.2 0.188

PartB

Table3: Readings for absorbance of a complex in the distilled water and atthe maximum wavelength of 480nm

 Time Absorbance at 480nm 0 0.251 5 0.234 10 0.183 15 0.151 20 0.127

Table4: Readings of absorbance of the complex found in acetone and at themaximum wavelength of 480

 Time Absorbance at 480nm 0 0.613 5 0.434 10 0.343 15 0.272 20 0.229

Discussion

Thepurpose of the experiment was to relate the concentration of lightabsorbing substance to the light absorbance quantitatively. Toachieve this purpose, the absorbance of all solutions was measured atthe maximum wavelength of 261nm. Using the concept of Beer LambertLaw, the absorbance of the unknown solution can be determined.Concentration of the unknown solution (0.00225 ml dm-3) is determinedby comparing the level of absorbance of that solution with thegradient in the graph light absorbance versus the concentration.

Thestock solution was used to facilitate the use of solutes in largemasses since they are easier to measure and handle. The stocksolution was also preferred because it is easier to store it comparedto the diluted solutions. In the present experiment, the stocksolution of 1cm3 was increased to 50 cm3 by adding cyclohexane toform solution X. About 1 cm3 of solution X was further increased to50 cm3 by adding about 50 cm3 of cyclohexane to for solution Y.

Theextent of light absorption was in the direct proportion with thelevel of analyte’s concentration. This means that when absorbingmolecules are more, more light is absorbed and less is transmitted.On the contrary, less light is absorbed when the level ofconcentration is low and more of it is transmitted since the numberof absorbing molecules is less.

Inpart B of the experiment, KSCN and Fe2 (SO4)3 were mixed withdistilled water while the second sample was mixed with acetone andexposed to the sunlight. It was observed that the level of absorbancewas the highest at time zero for the two mixtures. The followingreactions took place when these compounds were mixed

6KSCN (aq) + Fe2 (SO4)3(aq) → 6 K+ (aq) + 3 SO42-(aq) + 2 Fe (SCN)3(s)

[Fe(H2O) 6]3+ (aq) + SCN¯ (aq) ——&gt [Fe (SCN) (H2O) 5]2+ (aq) +H2O (l)

Thered color or the red precipate observed at the end of the reactionconfirmed the presence of iron compound, Fe (III) and the absence ofFe (II) (Science Discussion Board 1). The precipate was formedfollowing the paring of the anions (including thiocyanate andsulfate) and cations (including potassium and iron). Sunlightincreased the amount of energy available to increase the speed of thereaction (Banan 1). Aceton is considered to be the best ligandcompared to distilled water because it has a better light absorbingcapacity than distilled water (Transition metals 1). The experimentconfirmed the Beer-Lambert law since the level of absorbance wasdirectly proportional with the length of the path and theconcentration of the molecules of the solute.

Thespectrometer was closed properly during the experiment since failureto close might interfere with the accuracy of the results andprobability of the graph to cut through the origin.

Precautionsthat should be taken during the experiment

• Apparatus should be read from a position parallel to the eyes.

• A person handling the chemicals should wear groves

• The cuvette should be rinsed properly to avoid the contamination of the solution.

Conclusion

Thepresent experiment confirmed the Beer-Lambert law by showing that therate of absorption is directly proportion to the length of the pathand the concentration of the molecules of the analyte. This meansthat absorption is higher when the length of the path of longer andthe concentration of the molecules of the analyte is higher. In asimilar relationship, the level of absorption is lower when thelength of the path is lower and the concentration of the molecules ofthe analyte is lower.

Workscited

Banan,M. What is the effect of light, water, and acetone to reaction takingplace between potassium thiocynate and ferric sulphate? Yahoo.2015. Web. 19 July 2015.

DartmouthCollege. ChemLab:Spectronic 20.Hanover, NH: Dartmouth College, 2000. Web.

PVEducation Organization. Energy of photon. PVEducation Organization.2014. Web. 19 July 2015.

ScienceDiscussion Board. The art and science of amateur experimentatlism.ScienceDiscussion Board.2014. Web. 19 July 2015.

Transitionmetals. Reaction of some transition metals. Transitionmetals.Web. 19 July 2015.