Edexcel Jun 2012 Paper 1 Q25 (with explained solutions) |

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Edexcel Jun 2012 Paper 1 Q25

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25 Sodium burns in oxygen to give a pale yellow solid X. (a) (i) 1.73 g of sodium reacts with 1.20 g of oxygen. Calculate the empirical formula of X. (ii) The molar mass of X is 78 g mol1. Give the molecular formula of X. (2) (1)(iii) Write the equation, including state symbols, for the reaction of sodium with oxygen to produce X. (iv) Calculate the volume of oxygen in dm3 (at room temperature and pressure) which reacts with 1.73 g of sodium. (The molar volume of any gas at room temperature and pressure is 24 dm3 mol1.) (v) Calculate the number of oxygen molecules that react with 1.73 g of sodium. (The Avogadro constant = 6.021023 mol1.) (2) (2) (1) 18 *P39306A01824*<br />
(b) If sodium is burnt in air, compound X is not the only product. Suggest why this is so. (1)(Total for Question 25 = 9 marks) *P39306A01924* 19 Turn over<br />

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Question Number 25(a)(i) Amount Na = 1.73 (g)23 (g mol1) Acceptable Answers = 0.075(22) (mol) Amount O = 1.20 (g)16 (g mol1) = 0.075 (mol) (1) IGNORE sf, even if 1 sf NaO (1) Correct answer no working (2) NOTE: Correct answer can be obtained via incorrect working and all responses should be read carefully e.g. Amount Na = 231.73 = 13.3 Amount O = 161.20 = 13.3 scores second mark only for NaO if obtained by incorrect working OR e.g. Use of atomic numbers gives the Na : O ratio as 0.157 : 0.150 and an empirical formula of NaO. This scores (1) overall (i.e. the 2nd mark). OR e.g Use of atomic number ONLY for Na (i.e. Na = 11) gives the Na : O ratio as 0.157 : 0.075 and an empirical formula of Na2O. This scores (1) overall (i.e. the 2nd mark). NOTE: Use of O = 32 gives Na2O and scores second mark Question Number 25(a)(ii) (NaO = 39 hence molar mass twice Acceptable Answers Mark Reject Na2O2 Reject 2NaO Mark that of NaO ) so Na2O2<br />
Question Number 25(a)(iii) 2Na(s) + O2(g)Na2O2(s) Acceptable Answers All species correct (1) State symbols and balancing (1) Reject Mark NOTE: 2nd mark is conditional on correct species. NOTE: 2Na(s) + O2(g)2NaO(s) scores (1) Na(s) + O2(g)NaO2(s) scores (1) 4Na(s) + O2(g)2Na2O(s) scores (2)<br />
Reject Mark Reject Mark Question Number 25(a)(iv) Moles of O2 = 0.0752 = 0.0375 Acceptable Answers OR 1.232 = 0.0375 (mol) (1) 0.0375 mol x 24 dm3 mol1 = 0.9(0) (dm3) (1) ALLOW 900 cm3 (units must be present here) Correct answer no working (2) OR Moles of Na = 1.7323 = 0.075217 = moles of O Moles of O2 = 0.0752172 = 0.0376085 0.0376085 x 24 = 0.903 (dm3) or 903 cm3 IGNORE s.f., including ONE s.f. NOTE: If number of moles x 24 (dm3 mol-1) is clearly evident and correctly calculated in stated units, award second mark Question Number 25(a)(v) 0.0375 x 6.02 x 1023 Acceptable Answers (= 2.2575 x 1022 (molecules)) = 2.26 x 1022 (molecules) IGNORE s.f. unless 1 s.f.<br />
Mark Acceptable Answers Reject Sodium might react with nitrogen in the air/sodium forms a nitride/ nitrogen (gas) is present in the air (which reacts with the sodium) OR sodium might form a different oxide (e.g. Na2O or allow NaO2) NOTE: If nitrogen / N2 is mentioned as part of a list of substances that can be present in air, award the mark Just very reactive OR very explosive sodium forms Na2O2 alone References to hydrogen in the air Just reacts with other substances in the air (as nitrogen not identified Sodium nitrate formation Just sodium hydroxide formation Question Number 25(b)<br />

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