ons 3 and 4 refer to the absorption spectrum for a 0.40 M solu- tion of metal io
ID: 546703 • Letter: O
Question
ons 3 and 4 refer to the absorption spectrum for a 0.40 M solu- tion of metal ion Mt and a calibration curve for the metal ion Mt. Absorption Spectrum of 0.40 M M+ Calibration Curve for M+ 1.2 ---k-- 0.6--+--A--+ 0.4 + + + 0.4+-r-t- 0.3 0.2 400 450 500 550 600 650 700 (a) nm 0 0.1 0.2 0.3 0.4 0.5 M'1 (Molar) 3. At what wavelength was the calibration curve obtained? 4. A sample of this solution of unknown concentration of M* has an absorbance reading that is "off scale" when read at the wavelength that was used to construct the above calibration curve. You dilute the sample by adding 5.0 mL of water to 1.0 mL of the solution of M. The absorbance of the diluted sample when read at the wavelength of the calibration graph = 0.35. What is the concentration (M) of the original undiluted sample? Experiment 3-3 What Factors Affect Color Intensity? 21864204 uogdiosqvExplanation / Answer
3) The absorbance of a sample is usually measured at the wavelength of light where the sample absorbs the maxium, i.e, shows the plot of absorption vs wavelength shows a maxima at the wavelength of maximum absorption. Clearly, this wavelength is intermediate between 400 and 450 nm, i.e, approx.. 425 nm. Hence, the calibration curve must be obtained at 425 nm.
4) From the plot of absorbance vs concentration of M+, pick out the absorbance and the concentration values (these are given by the bold points on the plot).
[M+] Molar
Absorbance
0.1
0.20
0.2
0.35
0.3
0.50
0.4
0.70
Plot the graph again to get the regression equation.
Plot of absorbance vs [M+]
The regression equation is y = 1.65x + 0.025. Put y = 0.35 and find the concentration of M in the dilute sample.
0.35 = 1.65x + 0.025
====> 1.65x = 0.325
====> x = 0.19696 0.2
The concentration of M+ in the diluted sample is 0.2 M.
1.0 mL of the concentrated sample was taken and diluted with adding 5.0 mL water. The final volume is 6.0 mL and the dilution factor is (6.0 mL)/(1.0 mL) = 6.0.
Therefore, concentration of M+ in the concentrated sample = (0.2 M)*(dilution factor) = (0.2 M)*6.0 = 1.2 M (ans).
[M+] Molar
Absorbance
0.1
0.20
0.2
0.35
0.3
0.50
0.4
0.70
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