Question

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1a. To create the calibration curve a 5.00 cm pathlength cuvette was used in a spectrophotometer to measure the absorbance of a series of standard solutions. The wavelength used was 350 nm. The Beers law plot showed a linear function with the equation of y = 204.5x + 0.1.   If a sample of the same species with an unknown concentration is poured into a 1.00 cm pathlength cuvette and the resulting absorbance was 0.175, what would be the molarity of the analyte in the unknown sample?

1b. What is the molar absorptivity for the above species at a wavelength of 350 nm?   

2a. Light with an intensity of 10 mW/cm² impinges on a sample, and the intensity of the transmitted light is 5 mW/cm². What is the absorbance of this sample?   

2b. If the concentration of the absorbing species in the sample were doubled, what would be the new intensity of the transmitted light?   

Answer 1

1a,b

A(absorbance)=e.c.l

The plot of the absorbance vs concentration gives the straight line with the slope e.l (where e=molar absorbtivity coefficient,l=length of the cuvette)

the given equation has the slope of 204.5.Thus the e.l = 204.5 and e= 204.5/5 = 40.9 in mol-1Lcm-1(as l=5cm)

molarity is A/e.l = 0.175/40.9*1 = 0.0042 (in 1cm path length)

2a. Absorbance = log(I₀/I) where I₀ is the incident intensity and the I is the transmitted intensity of the light.

Thus A= log(10/5) = 0.698 .

2b.From the formula of the absorbance in the previous answer , If the concentration of the sample increases then the absorbance increases. In other words the intensity of the reflected/transmitted light from the sample will decrease more from the initial concentration thus in turn the absorbance of the sample increases.