A 0.500g sample is analyzed spectrophotometrically for manganese by dissolving it in acid and transferring to a 250 mL flask and diluting to volume. Three aliquots ae analyzed by transferring 50 mL portions with a pipet to 500 mL Erlenmeyer flask and reacting with an oxidizing agent, potassium peroxydisulfate, to convert the manganese to permanganate. After reaction, these are quantitatively transferred to 250 mL volumetric flasks, diluted to volume and measured spectrophotometrically.

By comparison with standards, the average concentration of the final solution is determined to be 1.25 x 10^-5M. What is the percent manganese in the sample?

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Collen VonLv2

6 Apr 2019

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Obtain a sample of steel. Rinse it with acetone to remove any oil and dry briefly. Weigh three samples of 0.5g of steel into 150 mL beakers.
Add about 50 mL of 6 M HNO3 to each sample and boil gently for 5-10 minutes. Then carefully add 1 gram of ammonium peroxydisulfate and boil gently for another
10-15 minutes. Add additional 6 M HNO3 if necessary to keep the solution volume at 30 - 50 mL.
Cool the solutions to room temperature, and transfer each to 250 mL volumetric flasks. Dilute to the mark with deionized water and mix well. Accurately transfer three 20.00 mL aliquots of each sample into individual beakers.
Treat them as follows:
Aliquot mL 85% H3PO4 mL std. Mn soln. grams KIO4

1 5ml 0.4g

2 5ml 0.4g 5ml

3 5ml 0.0g

Boil each of the treated aliquot solutions gently for 5 minutes.

Cool each solution to room temperature and quantitatively transfer each to 50 mL volumetric flasks. Dilute to the mark with deionized water and mix well.

When all three aliquots are ready, set the spectrophotometer at a wavelength of 525 nm, and measure the percent transmittance (to the nearest 0.1%) the aliquots.

Aliquot 1. 50.1%
Aliquot 2. 9.2%
Aliquot 3. 100.3%

Convert all transmittance data to absorbance, then calculate and report the percent manganese in each portion of the unknown analyzed,