2019 VJC H2 Chem Prelim P2 Ans

 VJC 2019 9729/02/PRELIM/19 [Turn over 1 Victoria Junior College 2019 H2 Chemistry Prelim Exam 9729/2 Suggested Answers 1 (a) An unknown solid sample, with a mass of 1.50 g, contains three sodium salts, NaC l, NaClO3 and NaNO3. The sample was completely dissolved in water and diluted in a 250 cm3 volumetric flask to obtain solution L. In one experiment, a 50 cm3 portion of solution L was reacted with excess silver nitrate solution. The AgCl precipitate formed was removed by filtration, dried and weighed. The AgCl precipitate was found to have a mass of 0.240 g. In another experiment, a gas was bubbled into a new 50.0 cm3 portion of solution L to convert ClO3 to C l. Excess silver nitrate was then added to the resulting mixture. The AgC l precipitated formed was treated similarly as before and found to have a mass of 0.285 g. (i) Determine the amount of ClO3- ions present in 50 cm3 of solution L. From 1st experiment, nAgCl = 0.240 107.9 + 35.5 = 1.674  10–3 mol = nCl- From 2nd experiment, nAgCl = 0.285 107.9 + 35.5 = 1.987  10–3 mol = nCl- + nClO3 - nClO3 - in 50.0 cm3 of L = 1.987  10–3 – 1.674  10–3 = 3.13  10–4 mol [2] (ii) Hence, determine the percentage by mass of NaClO3 present in the original solid sample. nClO3 - in 250.0 cm3 of L = (250 / 50)  3.13  10–4 = 1.565  10–3 mol = nNaClO3 Percentage by mass of NaClO3 = 1.565  10-3  (23.0 + 35.5 + 3  16.0) 1.50  100% = 11.1% [2] (b) Suggest whether NaCl or NaClO3 has a lower melting point. Explain your answer. NaClO3 has a lower melting point. Both Na Cl and Na ClO3 have giant ionic structure with electrostatic attraction between ions of opposite charges. │Lattice energy│∝ q+ × q- r++ r- . Since ClO3– is larger than Cl–, NaClO3 has a lower magnitude of lattice energy. Hence, less energy is required to break the ionic bonds. [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 2 (c) (i) Draw a dot-and-cross diagram of the ClO3– anion. [1] (ii) Chlorine forms a number of oxides, one of which is Cl2O7. Cl2O7 is a symmetrical molecule, with a central oxygen atom bonded to two chlorine atoms. Draw the structure of Cl2O7. [1] (iii) The bonds in Cl2O7 exhibit two different bond lengths, namely, 0.141 nm and 0.171 nm. However, all the bonds in the ClO3– ion have the same bond length of 0.149 nm. Suggest why the bond length in ClO3– is intermediate between those present in Cl2O7. Resonance arising from the overlapping of p orbitals of Cl atom with tho