RVHS H2 Chem 2012 Prelim P2 Soln

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 2 1 Planning (P) Calcium salts are used in many industrial processes. Calcium oxide is used in the production of cement while calcium nitrate is sometimes used to accelerate the setting of concrete. Calcium oxide is a white hygroscopic solid. Calcium oxide can react with water and as such can cause severe irritation when in contact with skin, eyes or if inhaled. Calcium nitrate is a lso a white hygroscopic solid. Calcium nitrate decomposes on heating to form the oxide, nitrogen dioxide and oxygen. (a) Write a balanced equation, including state symbols, for the thermal decomposition of calcium nitrate. Ca(NO3)2(s) ® CaO(s) + 2NO2(g) + ½ O2(g) [1] (b) Calculate the change in mass of the solid you would expect if 1.00 mol of calcium nitrate underwent thermal decomposition. Loss in mass = Mass of NO2 and O2 = 2(14.0 + 2(16.0)) + ½(2(16.0)) = 108 g [1] (c) A container of calcium nitrate was contaminated with calcium oxide. Design an experiment to determine the percentage purity of calcium nitrate by a gravimetric method. You are provided with about 2 g of the impure calcium nitrate. (In gravimetric methods, the o bjective is to form a thermally stable solid so that the mass can be determined accurately for analysis.) Your plan should include the following: · A full description of the procedure. · Details of how you would calculate the percentage purity from the measurements made. [Mr of Ca(NO3)2 = 164] 1. Weigh accurately a clean and dry crucible (Accept: hard glass tube). Add the sample and weigh the crucible with its contents. 2. Heat the crucible strongly for about five minutes to decompose the calcium nitrate. 3. Place crucible in a dessicator to cool. Then weigh the crucible and its contents. 4. Repeat the heating, cooling and weighing process until the mass of the crucible and its contents is consistent within 0.05 g in difference. Loss in mass = initial mass of crucible and contents – final mass of crucible and contents after final heating [8]

River Valley High School 9647/02/PRELIM/12 [Turn over Preliminary Examination 2012 3 Amount of Ca(NO3)2 = loss in mass / 108 Mass of Ca(NO3)2 = amount of Ca(NO3)2 ´164 % purity = mass of Ca(NO3)2 / initial mass of solid ´100% (d) State and explain clearly one precaution you would take to improve the accuracy of your results. Sample should be stored in dessicator while cooling to prevent absorption of water from the air. [1] (e) Identify one potential safety hazard in this experiment and state how you would minimise the risk. NO2 is toxic. Perform experiment in fume cupboard. OR CaO causes skin / eye irritation / irritation if inhaled. Wear gloves, mask etc. [1] [Total: 12] 2 Titanium(IV) oxide TiO2 is the naturally occurring oxide of titanium. It is the main i