A 0.2541±0.0001 g sample of a nickel alloy was dissolved in acid and diluted to
ID: 698124 • Letter: A
Question
A 0.2541±0.0001 g sample of a nickel alloy was dissolved in acid and diluted to 25.00±0.03 mL with distilled water. A 5.00±0.01 mL aliquot of this solution was then diluted to 50.00±0.05 mL with a complexing agent to form a colored complex with the nickel in the metal solution. This colored complex was analyzed by absorption spectrophotometry to obtain the nickel concentration in the final solution of 0.790 mg/mL, what is the % Ni in the original alloy sample and what is the uncertainty for this value? (20 points). The vitamin C content in a 25.00 mL sample of a fruit drink is analyzed by a coulometric titration using Iar as the titrant according to the reaction below. Is- is generated quantitatively between two platinum electrodes via the reactions below. A current of 25.00 mA requires 6 min and 17 s t reach an endpoint during this titration. What is the concentration of vitamin C in the fruit drink expressed in units of g/L? The molecular weight of vitamin C is 176 g/mol. CsHeOs +13. CsOcH6 + 2H + + 31. 12 + 1- 13.Explanation / Answer
The concentration of Ni in the diluted sample is 0.790 mg/mL.
The diluted sample was prepared by taking 5.00 mL of the initially prepared stock solution and diluting to 50.00 mL; the dilution factor is (50.00 mL)/(5.00 mL) = 10.00.
The concentration of Ni in the stock solution is (0.790 mg/mL)*(10.00) = 7.90 mg/mL.
0.2541 g sample was dissolved in 25.00 mL acid to prepare the stock solution. The mass of Ni in the stock solution prepared = (7.90 mg/mL)*(25.00 mL) = 197.50 mg = (197.50 mg)*(1 g/1000 mg) = 0.1975 g.
The wt% Ni in the original sample = (0.1975 g)/(0.2541 g)*100 = 77.7253% 77.725% (ans).
Uncertainty in the mass of the sample taken, U1 = (0.0001 g)/(0.2541 g) = 0.00039354.
Uncertainty in the preparation of the stock solution, U2 = (0.03 mL)/(25.00 mL) = 0.0012.
Uncertainty in the measurement of the aliquot, U3 = (0.01 mL)/(5.00 mL) = 0.002.
Uncertainty in the preparation of the dilute solution, U4 = (0.05 mL)/(50.00 mL) = 0.001.
Uncertainty in the wt% Ni, U = U12 + U22 + U32 + U42 = (0.00039354)2 + (0.0012)2 + (0.002)2 + (0.001)2 = 0.002568.
Absolute uncertainty in wt% Ni = U*(wt% Ni) = (0.002568)*(77.725%) = 0.1996% 0.20%.
The wt% Ni in the original alloy is 77.725 0.20% (ans).
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