Anchem 13/14 Q B2 Please show the answers clearly. A textile chemist determined
ID: 1071735 • Letter: A
Question
Anchem 13/14 Q B2
Please show the answers clearly.
A textile chemist determined the concentration of the dye DYE-X (dissolved in water) using UV/Visible spectrophotometry. A 5.0-mL portion of the dye solution was pipetted and diluted to 50.0 niL using a volumetric flask. This dye solution was placed in a quartz cuvette with a 2.5-cm light absorption path length, and its absorbance at 595 mil was measured to be 0.252. The absorbance of the blank (water) at 595 mil was also measured prior to the absorbance measurement on the dye solution. The molar extinction coefficient and molar mass of DYE-X are 28020 M^-1 cm^-1 and 380.2 g mol^-1, respectively. Briefly explain why the absorbance of the blank is measured and "zeroed" prior to the absorbance measurement on the dye solution. Calculate the transmittance and the concentration of DYE-X in the original sample (in ppm).Explanation / Answer
(i)
The absorbance of the blank is measured and zeroed to remove or deduct any signal from cuvette, solvent and dissolved gasses (if any) present in the sample.When you run your blank first and then your analyte, instrument subtracts spectrum of blank from the spectrum of the cuvette with analyte and you get a spectrum of just your analyte you are interested in or the sample present in the solution.
(ii)
l = 2.5 cm
A = 0.252
= 28020 M-1cm-1
m = 380.2 g/mol
c = ?
A = cl
c = A/l
c = 0.252 / 28020 M-1cm-1 x 2.5 cm = 3.59 x 10-6 M
c = 3.59 x 10-6 mol/l x 380.2 g/mol =1364.92 x 10-6 g/l = 1.364x10-3 g/l = 1.364 mg/l.= 1.364 ppm (1ppm = 1mg/L)
The original sample was diluted 50 times and hence the conc. in the oringal sample is 1.364 x 50 = 68.2 ppm
Calculation of Transmittance
A = 2-log(%T)
A = 0.252
log(%T) = 2-0.252 = 1.748
%T = 10(1.748) = 55.97
T = 0.5597
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