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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Friday, September 30, 2011

Questions About Aluminium's Chemical Compounds

Aluminium is the most abundant metal in the Earth‘s crust. Some compounds of aluminium, like alum (the double sulphate KAl(SO4)2·12H2O) were in use since ancient Greece time.

a) The freezing point depression constant for water is Kf = 1.86 °C kg mol-1. Calculate the freezing point of the solution prepared by dissolving 9.48 g of KAl(SO4)2·12H2O in 100.0 g of water.

b) The most important ore of aluminium is bauxite, which consists from Al2O3 and impurities. Bauxite can be purified by dissolving in concentrated NaOH solution. Write molecular equation that shows what is happening when Al2O3 reacts with NaOH solution.

c) After removal of insoluble impurities, weak acid is added to the solution obtained in part b and Al(OH)3 is precipitating. Solubility product KSP for Al(OH)3 is 1.3 x 10–33 (at 25 °C). Calculate mass of pure Al(OH)3 that can be dissolved in 10.0 L of pure water at 25 °C.

EDTA (ethylenediaminetetraacetic acid or its salts) is among the most important and widely used reagents in titrimetry. Unfortunately, this reagent is not good for direct determination of aluminium ions, because EDTA reacts with aluminium ions too slowly. Problem can be solved by applying back titration method. In the beginning EDTA and aluminium compound solution are allowed to react in the hot solution for several minutes, and then excess EDTA can be fast and easily titrated with Zn2+(aq). Analysis is performed in presence of buffer solution. Reactions taking place in this method can be represented as:

Al3+ + EDTA4– à Al·EDTA and Zn2+ + EDTA4– à Zn·EDTA2–

d) 20.00 mL of solution, that contains Al3+, was transferred into flask that contains 50.00 mL 0.0500 M EDTA solution. Mixture was boiled for several minutes. For titration of the resulting mixture 23.25 mL of 0.0500 M Zn2+ solution was necessary. Calculate molar concentration of Al3+ in the initial solution.

Trihalides of aluminium (like AlCl3) are very strong Lewis acids and finds extensive use as catalysts. With some ligands L aluminium trichloride may form four coordinated AlCl3L or even five-coordinated AlCl3L2.

e) Assuming that L is a monodentate ligand, draw all possible geometrical isomers of AlCl3L2.

f) In gas phase aluminium trichloride exists as Al2Cl6 molecules with two bridging Cl atoms. Make a drawing that represents spatial structure of Al2Cl6 molecule.

Copyright by Friedrich Rabin Situmorang

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