Edexcel Jan 2012 Paper 1 Q20 (with explained solutions) |

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Edexcel Jan 2012 Paper 1 Q20

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 20 (a) An impure sample of sodium hydrogencarbonate, NaHCO3, of mass 0.227 g, was reacted with an excess of hydrochloric acid. The volume of carbon dioxide evolved was measured at room temperature and pressure and found to be 58.4 cm3. NaHCO3 + HCl(cid:3)(cid:111)(cid:3)NaCl + H2O + CO2 The molar volume of any gas at the temperature and pressure of the experiment is 24 dm3 mol1. The molar mass of sodium hydrogencarbonate is 84 g mol1. (i) Calculate the number of moles of carbon dioxide given off. (ii) Calculate the mass of sodium hydrogencarbonate present in the impure sample. (iii) Calculate the percentage purity of the sodium hydrogencarbonate. Give your answer to two significant figures. PMT (1) (2) (2) 10 *P39295A01024*<br />
(b) (i) The total error in reading the gas syringe is 0.4 cm3. Calculate the percentage error in measuring the gas volume of 58.4 cm3. (ii) Suggest why the carbon dioxide should not be collected over water in this experiment. PMT (1) (1)(Total for Question 20 = 7 marks) *P39295A01124* 11 Turn over<br />

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Section B Question Number 20(a)(i) Acceptable Answers Reject (Amount CO2 = 0.0584 dm324 dm3 mol1) = 0.0024333/2.4333 x 103 (mol) IGNORE sf except 1 No working needed Mark final answer 0.002/2 x 103 or any other value WRONG units with correct numerical answer scores (0) Mark Acceptable Answers Question Number 20(a)(ii) First mark: Reject Mark amount CO2 = amount NaHCO3 OR use of candidates answer to (a)(i) stated (or implied by final answer given) (1) Second mark: = 0.2044 (g)mass NaHCO3 = 0.0024333 (mol) x 84 (g mol1) ALLOW 0.2 (g) This mark is for evidence of multiplying their moles of NaHCO3 by 84 (1) IGNORE sf including 1 sf Correct answer with no working scores (2) ALLOW consequentially from (i). 6CH0101 1201<br />
Acceptable Answers Question Number 20(a)(iii) % purity = (0.2044 g x 100)0.227g = 90.04 % (1) = 90% (1) (2 sf only) ALLLOW consequentially from (i) and (ii) NOTE: The second mark to be awarded for 2sf answers less than a 100% (e.g. 10% scores (1). This is the percentage impurity) Correct answer with no working scores (2) Can score both marks via moles rather than masses Acceptable Answers Question Number 20(b)(i) 0.4 / 58.4 x 100 = () 0.68493(%) IGNORE sf (including 1 sf so () 0.7 (%) is OK here) Acceptable Answers Question Number 20(b)(ii) Any one of:- CO2 dissolves /soluble (in water) CO2 reacts (with water) / CO2 forms carbonic acid / CO2 + H2OH2CO3 ALLOW CO2 absorbed (by water) IGNORE suggestions to use a gas syringe Mark Reject Answers not to 2 sf or answers incorrectly rounded up do not score 2nd mark Answers > 100% score (0) overall Mark Mark Reject () 1.37 (%) etc., as the uncertainty should NOT be doubled Answers incorrectly rounded (e.g. 0.684 / 0.67 / 0.68492) Reject CO2 not the only gas given off CO2 diffuses/is lost/mixes with water Water is also a product of the experiment Suck-back 6CH0101 1201<br />

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