OCR A Jun 2011 Paper 1 Q2

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2 Magnesium is the eighth most abundant element in the Earths crust and many rocks are a source of magnesium compounds. Magnesium carbonate, MgCO3, is present in dolomite, a rock found in the Dolomite mountains in Italy. A student collected two equal-sized samples of dolomite. These samples were put into two labelled test-tubes, A and B. Tube A was heated until there was no further change in mass and was then allowed to cool. Tube B was left unheated. (a) Write the equation for the action of heat on the magnesium carbonate present in tube A.[1] (b) The student wanted to make magnesium chloride crystals. The student added an excess of warm dilute hydrochloric acid to tube A and to tube B. (i) Write the equation for the reaction of magnesium carbonate in tube B with dilute hydrochloric acid. Include state symbols.[2] (ii) State one similarity and one difference the student would see between the reactions in the two tubes. similaritydifference[2] (iii) From the solution in each tube, the student obtained crystals with the formula MgCl26H2O. Calculate the relative formula mass of MgCl26H2O. Give your answer to one decimal place. relative formula mass =[1]OCR 2011<br />
 (iv) Draw a dot-and-cross diagram to show the bonding in MgCl2. Show outer electrons only. (c) A compound containing magnesium, silicon and oxygen is also present in rock types in Italy. A sample of this compound weighing 5.27 g was found to have the following composition by mass: Mg, 1.82 g; Si, 1.05 g; O, 2.40 g. [2] Calculate the empirical formula of the compound. Show your working. empirical formula =[2]OCR 2011 Turn over<br />
 (d) Pharmacists sell tablets containing magnesium hydroxide, Mg(OH)2, to combat indigestion. A student carried out an investigation to find the percentage by mass of Mg(OH)2 in an indigestion tablet. The student reacted the tablet with dilute hydrochloric acid. Mg(OH)2(s) + 2HCl(aq) MgCl2(aq) + 2H2O(l) The student found that 32.00 cm3 of 0.500 mol dm3 HCl was needed to react with the Mg(OH)2 in a 500 mg tablet. [1 g = 1000 mg]. (i) Calculate the amount, in mol, of HCl used. (ii) Determine the amount, in mol, of Mg(OH)2 present in the tablet. amount =mol [1] (iii) Determine the percentage by mass of Mg(OH)2 present in the tablet. amount =mol [1] answer =% [3] [Total: 15]OCR 2011<br />

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