Question

A, What does the UV-Vis spectrophotometer measure? What is Beer’s Law (you might have to look it up) and how does it relate to data collected from a spectrophotometer?  

B, What is a sample blank? What it is it used for in this experiment?

C, What does the addition of NH3(aq) do to the copper(II) solution? If you don’t know because you haven’t yet done the experiment, make an educated guess. (Think about what the NH3 does chemically and what a spectrophotometer measures analytically.)

D, In your experiment, which is the independent/dependent variable? When you make your calibration curve, which variable will be on the x-axis? the y-axis?

E, How will the unknown concentration of Cu2+ be determined in your unknown?

F, What does it mean to dilute a sample? Explain how you would make 10.00 mL of a 0.03 M Cu2+ solution from a standard solution of 0.10 M CuSO4.

plz write your answer clear. thank you

Answer 1

Ans. A

UV-Vis spectrophotometer measures the difference between intensity of light before passing through sample and intensity of light passed throgh sample which gives light absorbed by sample.

Beer's law says that the light absorbed by the sample is direcctely propertional to concentration of sample and also directly proportional to the pathlength of the sample. (width of cuvette).

,

where, A = Light absobed by sample l= pathlength of sample

   $\epsilon$ = extinction coefficient, c= concentration of sample

Spectrophotometer gives absorption by sample and if concentration and pathlength is already known then, we can able to draw calibration curve, from which we can find the concentration of unknown sample by measuring its absorption from spectrophotometer and relate that value to the calibration curve.

Ans. B

Blank solution is solution which contains no any interested analyte and also the solvent from which samples are made. Blank solution is required in spectrphotometer to calibrate it and also the absorption spectra of blank sample is taken as baseline for our analyte samples.

Ans C.

The addition of ammonia in copper solution forms water complex which colour is dark blue. The colour of Cu(II) solution is slightly blue but addition of NH₃ forms [Cu(NH₃)₄]²⁺ complex which is dark in coloured. So that absorption of light is increased due to dark blue colour.

Ans D.

In experiment, absorption is dependent varible on the concentration of solution and wavelength of light passed through sample. where as width of cuvette is constant so that it will not affect absorption. In calibration curve, Absorption is on Y-axis where as concentration of sample is on X-axis.

Ans E.

By measuring the absorption of unknown sample, then relate the absorption value with concentartion by putting that values on calibration curve. we can easily predict the conc. of unknown sample.

Ans F.

Beer's law is only applicable on dilute ssolution. Dilute solution means it contains very less amount of analyte or solute in solution or its concentration is low.

To make 10 mL of 0.03 M solution of Cu²⁺ from 0.10 M by following method.

M₁V₁ = M₂V₂

where, M₁ = 0.10 M Cu²⁺ solution V₁ = Vol. of solution requred of 0.1 M Cu²⁺ solution

M₂ = 0.03 M Cu²⁺ solution V₂ = 10 mL

V₁ = $V_{1} = \frac{0.03\ast 10}{0.1} =3mL$

Take 3 mL of 0.1Cu²⁺ solution and add water upto 10 mL. The resultant solution becomes 0.03 M