1. Aluminum is determined gravimetrically by precipitating Al(OH)3 and isolating Al½O3. Asample that contains approximately 0.1 g of Al is dissolved in 200 mL of H2O, and 5 g of NH4C1and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 andyellow at pH levels above 6. The solution is heated to boiling and 1:1 NH3 is added dropwise untilthe indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution's boilingpoint for several minutes before filtering and rinsing with a hot solution of 2% w/v NHẠNO3. Theprecipitate is then ignited at 1000–1100 °C, forming Al>O3.a) Cite at least two ways in which this procedure encourages the formation of larger particlesof precipitate.b) The ignition step is carried out carefully to ensure the quantitative conversion of Al(OH)3 toAl203. What is the effect of an incomplete conversion on the %w/w Al?c) What is the purpose of adding NHẠCI and methyl red indicator?d) An alternative procedure for aluminum involves isolating and weighing the precipitate as the8-hydroxyquinolate, Al(C,H,NO)3. Why might this be a more advantageous form of Al for agravimetric analysis? Are there any disadvantages?

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1. Aluminum is determined gravimetrically by precipitating Al(OH)3 and isolating Al½O3. A sample that contains approximately 0.1 g of Al is dissolved in 200 mL of H2O, and 5 g of NH,Cl and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 and yellow at pH levels above 6. The solution is heated to boiling and 1:1 NH3 is added dropwise until the indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution's boiling point for several minutes before filtering and rinsing with a hot solution of 2% w/v NHẠNO3. The precipitate is then ignited at 1000–1100 °C, forming Al>O3.

1. Aluminum is determined gravimetrically by precipitating Al(OH)3 and isolating Al½O3. A sample that contains approximately 0.1 g of Al is dissolved in 200 mL of H2O, and 5 g of NH,Cl and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 and yellow at pH levels above 6. The solution is heated to boiling and 1:1 NH3 is added dropwise until the indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution's boiling point for several minutes before filtering and rinsing with a hot solution of 2% w/v NHẠNO3. The precipitate is then ignited at 1000–1100 °C, forming Al>O3.

Chemistry

10th Edition

ISBN:9781305957404

Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Chapter1: Chemical Foundations

Section: Chapter Questions

Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...

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Question

1. Aluminum is determined gravimetrically by precipitating Al(OH)3 and isolating Al½O3. A
sample that contains approximately 0.1 g of Al is dissolved in 200 mL of H2O, and 5 g of NH4C1
and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 and
yellow at pH levels above 6. The solution is heated to boiling and 1:1 NH3 is added dropwise until
the indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution's boiling
point for several minutes before filtering and rinsing with a hot solution of 2% w/v NHẠNO3. The
precipitate is then ignited at 1000–1100 °C, forming Al>O3.
a) Cite at least two ways in which this procedure encourages the formation of larger particles
of precipitate.
b) The ignition step is carried out carefully to ensure the quantitative conversion of Al(OH)3 to
Al203. What is the effect of an incomplete conversion on the %w/w Al?
c) What is the purpose of adding NHẠCI and methyl red indicator?
d) An alternative procedure for aluminum involves isolating and weighing the precipitate as the
8-hydroxyquinolate, Al(C,H,NO)3. Why might this be a more advantageous form of Al for a
gravimetric analysis? Are there any disadvantages?

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