ACJC_H1_CHEM_P2_ans

2 © ACJC 2017 JC2 H1 Preliminary Exam 2017 Section A Answer all questions in this section in the spaces provided. 1 Aluminium is the most abundant Group 13 element and constitutes about 8% of the Earth’s crust. The extraction of aluminium is done by processing aluminium ore, bauxite to produce aluminium oxide also known as alumina. A variety of aluminium compounds, for example aluminium chloride and aluminium hydroxide, are used for different purposes such as food additives, colouring and pharmaceuticals. Aluminium hydroxide and magnesium hydroxide are antacids. They are used to treat symptoms of increased stomach acid, such as heartburn, upset stomach, sour stomach, or acid indigestion. Once ingested, they react with the hydrochloric acid in the stomach. One label of a commercial product, Mintox™ is shown below. (a) (i) Write down the electronic configuration of Al. [1] 1s22s22p63s23p1 (ii) Why is the ionic radius of aluminium far smaller than its atomic radius? Al 3+ ion has a higher proton to electron ratio, hence the remaining electrons experience stronger electrostatic forces of attraction to the nucleus. In addition, Al 3+ ion has one less principal quantum shell as compared to the neutral atom, hence the ionic radius is smaller than its atomic radius. (iii) Explain why aluminium forms compounds with an oxidation state of +3 but not sodium. [1] As compared to sodium, aluminium would require smaller amount of energy to remove the (2 nd and 3rd) electrons which are from the outermost principle quantum shell.

3 © ACJC 2017 JC2 H1 Preliminary Exam 2017 (b) (i) Which antacid in the tablet is more effective in reacting with the hydrochloric acid in the stomach? Show relevant working to support your answer. [2] Relevant working based on masses in the tablet No of moles of Mg(OH) 2 = 200 x 10-3 / 58.3 = 3.43 x 10-3 mol No of moles of Al(OH)3 = 200 x 10-3 / 78 = 2.56 x 10-3 mol Al(OH)3 will produces more number of moles of hydroxide ions (7.68 x10- 3) than Mg(OH)2(6.86 x10-3) . Thus aluminium hydroxide is more effective. (ii) Calculate the maximum number of chewable tablets that a person can take in a week. [1] 4x 4 x 7 = 112 (iii) Assuming that a typical adult has a body mass of 70 kg, determine the maximum weekly intake of aluminium hydroxide in grams per kg of body mass. [2] Maximum intake of aluminium hydroxide is 200 mg x 112 Maximum intake per body mass = 0.320 g per kg (c) (i) Aluminium chloride is an active ingredient used in skin medication to control excessive sweating. Aluminium chloride is often describe as electron