Question

PROBLEM: Predict the molar absorptivity value of copper sulfate pentahydrate at 380 nm.

Here's what I think I have to do:

I think I need to use beer's law. Abs=ebc; e=Abs/bc.

b is 1.66 cm. I think c is .400 M NiSO4, but I'm not sure. I also think Abs is 0.758 given the graph. (If wavelength is 380 nm, then it should be between 0.598 and 0.798, right?) My answer is 1.14 L/mol*cm, but not sure if I am completely doing it wrong.

3. Solving for [Ni(11)] and [Cu(I)] in an unknown Solution In this experiment, you will first establish the newly defined constant k (molar absorptivities x path lengths), for Cu2+ and Ni2+ at two analytical wavelengths. You will then determine the concentrations of nickel and copper in an unknown mixture. Examine Figure 8.1. In this plot of Absorbance versus Wavelength (nm), we can clearly see near the peak absorbance wavelength of Cu2+ (770 nm), that there is also appreciable absorbance by Ni2+; however, at the peak absorbance wavelength of Ni2+ (380 nm), there is no absorbance by Cu2+. 1.198 0.998 0.798 --Ni(1) Abs 0.598 F ---Cu(ll) 0.398 0.198 E -0.003 332 382 432 ecce 482 532 582 632 Wavelength (nm) 682 732 782 Figure 8.1 Absorbance spectra for Cu (II) and Ni (II). Therefore, the previously derived system of equations can be used to solve for both unknowns: [Ni2+] = _ A380 K380.Nia- [Cu²+1= A/70 - kcom[Ni2+] 5770.Cu

Answer 1

The problem specified is predict the molar absorptivity of copper sulphate pentahydrate at 380 nm.
To predict the molar absorptivity you can use Beer Lambert Law. which is
absorbance = molar absorptivity * Constant
constant is path length * molar concentration
In the figure itself it is clear that You have no absorbance at 380 nm for the cupper sulphate hence the value of absorbance or molar absorptivity at this wavelength also will be zero.