Question

The good chemist is tasket with determining the concentration of manganese for a water sample drawn from a well next to a mine. To analyze the sample using absorbance spectroscopy, she prepares six sample (0-5) by adding 10.00 mL well water to 50.00-mL volumetric flasks, then adding increasing volumes of 25.00 ppm maganese solution, and diluting the resulting solution to 50.00 mL with 0.1 M HNO3. Final analysis at lambda(max)= 280 nm produced a graph of absorbance vs ppm Mn with best-fit line y = 0.0428x + 0.0487.

a) What is the concentration in ppm of manganese in the well water sample?

b) A field method of analysis was developed for testing soil near the mine where a .570 g sample of soil is quickly dissolved in acid with an oxidizing agent and diluted to 100 mL.

i) While you wait for the extraction, you calivrate your USB-spectrometer with a 3.15x10^-4 M standard of MnO4- with an average absorbance of .407 at 565 nm in a 1 cm cuvette. A blank solution made from DI water and acid had an absorbance of .078. Find the molar absorptivity of permanganate complex

ii) A few mL of the unknown solution is placed in a 1 cm cell and the transmission is found to be 30% at 565 nm. Calculate the concentration of manganese in the extracted sample.

iii)What was the weight percent of manganese in the original soil sample. iv) Explain in one sentence why permanganate appears to be a purple color

Answer 1

(a) In the best fit line equation y = 0.0428x+0.0487, the intercept reperent the concentration of Mn already present in the water sample and no permanganate is added from outside (x=0). At x= 0 , as no permanganate is added externally, the absorbance is due to the the Mn already present in the water.

slope of this line reperent the molar absorptivity.

A = 0.0487 and = 0.0428

C = 0.0428/0.0487 = 0.879 ppm

0.879ppm Mn is present in 50mL solution. So, in the 10mL sample amount present = 0.879/5 = 0.176 ppm

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(b) total absorbance = absorbance due to MnO4^- + absorbance due to water

or, absorbance due to MnO4^- =  total absorbance - absorbance due to water

or, absorbance due to MnO4^- = 0.407-0.078 =0.329

0.329 = 3.15x10^-4 M *   * 1cm

or,   =1044.44

molar absorptivity is 1044.44 L/mol/cm

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(ii) Concentration of Mn in the extracted sample :

Absorbance = 2-logT% = 2-log30 = 0.52

0.52 = 1044.44L/mol/cm * C*1cm

or, c = 4.98*10^-4 M = 4.98*10^-4 moles/L

amount of Mn present in 100mL = 4.98*10^-5 moles

mass of Mn present in 100mL = 4.98*10^-5 moles * 54.94gm/mol =0.0027 gm

weight % = 0.0027*100/0.57 = 4.74%

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MnO4- has a srong purple color due to O2- to Mn ^7+ chanrge transfer