Name: Lab 10 Postlab Questions What color is the 1. The absorbance spectrum of a
ID: 560186 • Letter: N
Question
Name: Lab 10 Postlab Questions What color is the 1. The absorbance spectrum of an unknown solution is shown below. solution? 1.2 630 m 0.8 0.6 0.4 0.2 400 600 800 Wavelength (nm) Astudent was using spectroscopy to find the concentration of a solution of compound X. She first made a series of known dilutions, measured their absorbances, and made the Beer's Law plot shown below. (Recall that displaying an number in the form 5.0E-06 is computer talk for scientific notation, so we would write this number as 5.0 x 106) 2. Beers Law Plot of Cmpd X 14 1.2 +0.0209 08 a 0.6 0.4 0.2 R 0.9974 .0E+00 5E-06 LOE-05 1 SE-05 2 OE-05 2.5E-05 X] (M) She then measured the absorbance of her unknown solution to be 0.878. a) What is the extinction coefficient of compound x? b) What is the concentration of her unknown solution?Explanation / Answer
1) The solution shows a maximum absorbance at 630 nm; we need to refer to a chart which displays the color of a substance against the wavelength of absorption (easily available in labs). A substance absorbs maximum at 630 nm when it is yellow in color; hence, the solution is yellow.
2a) The student obtained the linear regression equation as y = 63629x + 0.0209 where y is the absorbance of the solution and x is the concentration of the solution. x has unit M; however, y is dimensionless. In order to make y dimensionless, we must have unit M-1 with the slope. The slope of the plot is gives the product of the extinction co-efficient and the path length of the solution, i.e,
63629 M-1 = *l where l = 1.00 cm is usually taken as the path length of the solution and is the extinction co-efficient; therefore,
= (63629 M-1)/(1.00 cm) = 63629 M-1cm-1 (ans).
b) The unknown solution shows an absorbance 0.878; put y = 0.878 and obtain
0.878 = (63629 M-1)*x + 0.0209
====> (63629 M-1)*x = 0.8571
====> x = 0.8571/(63629 M-1) = 1.34702*10-5 M 1.35*10-5 M
The concentration of the unknown solution is 1.35*10-5 M (ans).
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