Serial Dilution of Pota ssium Pema Spectroscopy Pemanganate using UV-Vis Chemist
ID: 716252 • Letter: S
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Serial Dilution of Pota ssium Pema Spectroscopy Pemanganate using UV-Vis Chemistry 201 Section Name Pre-labora tory Questions 1. Define the folowing terms a· Serial Dilution b. UV-Vis Spectroscopy c. Beer'sLaw 2 Determine the molarity (mM) for a 0.33 M solution of NasFO4 to 1.00 L solution of Na FO. prepared by dluted 100 mlof a 3. 5.00 mLof the 1.0 Lsolution prepared in question 2 wasdiuted to 50.0 mL, what woud be the concentration (mM) of Na PO in this new solution? 4. The absorbance of a 0.100 mM solution of a dye in a 1.00-cm cell s0.982 at 420 nm. ulate the molar ab sorptivity constant (M-'cm") for this dye. 5. What would be the absorbance 0.0500 mM a 1.00-cm cell is used for this mea surement. ance of a 0.0500 mM solution of this dye at 420 nm? Assume Revised 06/2016 CNExplanation / Answer
1a) A serial dilution is a stepwise dilution to a substance to prepare solutions containing a lower concentration of the original substance. The dilution factor is kept constant so that the concentration of the first solution is an exact multiple of the second solution; the concentration of the second and the third solutions bear the exact same ratio as the concentrations of the first and the second solutions. Say we wish to prepare a series of solutions by serial dilution starting from a 1 M solution. Suppose, we wish to dilute the solutions 10-fold, i.e, the dilution factor is 10. Then the concentrations of the solutions will vary as
Solution 1 = 1 M (supplied)
Solution 2 = (1 M)/10 = 0.1 M
Solution 3 = (0.1 M)/10 = 0.01 M
Solution 4 = (0.01 M)/10 = 0.001 M
and so on.
1b) Ultra-violet spectroscopy refers to the absorption of light in the ultraviolet and visible ranges of the electromagnetic spectrum. The wavelength of the light absorbed ranges from 400-800 nm. Ultraviolet spectroscopy is highly useful to detect the presence of unsaturations in a compound.
1c) Beer’s law is an empirical law that governs the absorbance of light by a solution of a compound and relates the absorbance quantitatively to the concentration of the substance in the solution. The Beer’s law is expressed as
A = *C*l
where A = absorbance of the solution containing the compound having concentration C and l is the path length of the solution through which the light passes. is a constant of proportionality and is known as the molar absorptivity of the substance and is unique for a particular substance.
2) Use the dilution equation:
C1*V1 = C2*V2
where C1 = concentration of the stock solution; V1 = volume of the stock solution; C2 = concentration of the diluted solution and V2 = volume of the diluted solution.
We have C1 = 0.33 M, V1 = 10.0 mL and V2 = 1.00 L = (1.00 L)*(1000 mL)/(1 L)
= 1000 mL.
Plug in values and obtain
(0.33 M)*(10.0 mL) = C2*(1000 mL)
====> C2 = (0.33 M)*(10.0 mL)/(1000 mL)
====> C2 = 0.0033 M = (0.0033 M)*(1000 mM)/(1 M)
= 3.3 mM (ans).
3) We take 5.0 mL of the solution prepared in step 2 and dilute to 50.0 mL. Use the dilution equation again.
C1*V1 = C2*V2
where C1 = 3.3 mM; V1 = 5.0 mL and V2 = 50.0 mL.
Plug in values and obtain
(3.3 mM)*(5.0 mL) = C2*(50.0 mL)
====> C2 = (3.3 mM)*(5.0 mL)/(50.0 mL)
= 0.33 mM (ans).
4) Use Beer’s law:
A = *C*l
=====> = A/Cl
Given A = 0.982 at 420 nm, C = 0.100 mM and l = 1.00 cm, we have
= (0.982)/(0.100 mM)(1.00 cm)
= (0.982)/[(0.100 mM)*(1 M)/(1000 mM)*(1.00 cm)]
= 9820 M-1.cm-1 (ans).
5) It has been stated that is unique for a substance and depends only on the identity of the substance. We have = 9820 M-1.cm-1.
Use Beer’s law.
A = *C*l
= (9820 M-1.cm-1)*(0.0500 mM)*(1.00 cm)
= (9820 M-1.cm-1)*(0.0500 mM)*(1 M)/(1000 mM)*(1.00 cm)
= 0.491 (ans).
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