Question

A student is instructed to determine the concentration of a solution of CoCl2 based on absorption of light (spectrometric/colorimetric method). The student is provided with a 0.10 M solution of CoCl2 with which to prepare standard solutions with concentrations of 0.020 M, 0.040 M, 0.060 Mand 0.080 M 

(a) Describe the procedure for diluting the 0.10 M solutions to a concentration of 0.020 M using distilled water, a 100 mL volumetric flask, and a pipet or buret. Include specific amounts where appropriate. 

The student takes the 0.10 M solution and determines the percent transmittance and the absorbance at various wavelengths. The two graphs below represent the data. LOO-t so 6O 2O 360 MOO 44O 48O 520 560 60DO O.60 O. to O.2o ALSO 52 SSO wavelength Cnm) 

(b) Identify the optimum wavelength (Amax) for the analysis.

Answer 1

Ans a) Here it is given

M₁=0.10M

M₂=0.020M

V₁= x ml

V₂= 100ml

Now M₁V₁ =M₂V₂

0.10 x (x) = 0.020 x 100

x=20ml

Now for diluting the 0.10M solution to a concentration of 0.020M.We will measure 20ml of 0.10M solution and pipet it to a volumetric flask of 100ml. After that we will add distilled water to the solution contained in volumetric flask.

b) For identifying the optimum wavelength we wii take the absobance v/s wavelength graph as the wavelength at which absorbance is highest is the optimum wavelength.

timal o 60 510nm o 40 0-20

From the graph it is clear that at 510nm the absorbance is highest so it is the optimum wavelength.

d) The expression for Beers law is

A=$\epsilon lC$

Here $\epsilon$ is the molar absorpvity constant

l= path length or the distance travelled by light in the solution

c is the concentration of the solution

e) The calculated concentration wil be high than the actual concentration as the student's fingerprints absorbs the light.

f) The same method for measuring the concentration of NaCl solution would not be appropriate as NaCl is colourless and it will transmit light.