Chemical Formulae, Equations, Calculations

Transcription

1 Chemical Formulae, Equations, Calculations Mark Scheme 3 Level IGCSE(9-) Subject Chemistry Exam Board Edexcel IGCSE Module Single Award (Paper C) Topic Principles of Chemistry Sub-Topic Chemical Formulae, Equations, Calculations Booklet Mark Scheme 3 Time Allowed: 59 minutes Score: /49 Percentage: /00 Grade Boundaries: >90% 80% 70% 60% 50% 40% 30% 0% 0%

2 Answer Notes Marks (a) M iron reacted with oxygen Accept iron combined/bonded with oxygen Accept iron oxide formed Accept iron is oxidised Ignore iron uses oxygen Ignore iron rusts Ignore references to reacting with water M all oxygen is reacted / (all) oxygen used up / no oxygen left Accept references to 0% or 0cm 3 of the air which is oxygen used up/reacted Reject all iron used up Ignore reaction has finished (b) M iron(ii) sulfate / iron sulfate reject any other oxidation state M hydrogen

3 (c) M (Fe + ) green precipitate/solid ignore shades reject other colours eg bluegreen M (Fe 3+ ) brown precipitate/solid accept red-brown / orange brown Ignore rust coloured reject red on its own Allow mark if both answers correct but reversed Ignore references to colours of solutions

4 (a) Answer Notes Marks M (mol NaHCO 3 =) 0.5/84 or 0.5 M (so mass CO = x 44 = ).8 (g) correct final answer with no working scores accept.75 M CQ on M OR M 68 g NaHCO 3 give 44 g CO M 0.5 g NaHCO 3 give.75 g CO (b) M (mol CO =) or correct final answer with no working scores if answer is incorrect mark CQ to (a) M (0.065 x 4000) = 500 (cm 3 ) CQ answer to M accept.5(00) dm 3

5 Answer Notes Marks 3 a i carbon monoxide ii decreases capacity of blood (cells) to carry oxygen OR stops blood (cells) from carrying oxygen Accept CO combines with haemoglobin / forms carboxyhaemoglobin Accept CO displaces/replaces oxygen in haemoglobin Ignore CO combines with red blood cells Ignore references to suffocation / lack of oxygen in lungs stopping breathing / gas exchange Ignore just affects haemoglobin Reject destroys haemoglobin b i 6KClO 3 + S + P 4S 3 6KCl + 4SO + P 4O 0 M M coefficient of 6 for KCl coefficient of 4 for SO Max mark if equation unbalanced Ignore for other coefficients 0 for other coefficients loses M ii activation (energy) Total 5 marks

6 4 (a) (i) 08/4 = 4.5 Expected answer Accept Reject Marks mark for answer of 4.8() (molar volume =.4dm 3 ) (ii) M r of NaN 3 = (4 x3) Moles of NaN 3 = 3 OR two thirds of (a)(i) Mass of NaN 3 = 95 (g) OR moles of NaN 3 x M r [Mark consequentially at each stage] Correct answer with no working scores 3 (b) (i) Removes (harmful) sodium Produces more nitrogen / gas OR bag inflates more quickly (ii) K O(s) + SiO (s) K SiO 3 (s) OR K O(s) + SiO (s) K SiO 3 (l) IGNORE same s of Na O on both sides of equation (c) (i) (ii) Precipitation Filtration / filter IGNORE refs to adding water Double decomposition Decanting / pour off liquid Double displacement Sieving / evaporation / distillation / crystallisation / heat Total 9 Marks

7 5 (a) (i) M - Answer Accept Reject Marks One mark for (44 4) = 6 M (ii) (iii) M -.. M - 48 (MUST be a whole ) (iv) M - (CO 3 ) = 60 M - 88 M3 - Sr / strontium Mark csq throughout part (a) answer csq on correctly calculated value of M (i.e. metal closest to calculated A r ), but must be a Group metal

8 Number 5 (b) Any two from: Answer A Reject Marks M - gas was lost between adding acid and replacing bung M - bung does not fit/there are leaks in the apparatus M3 - some gas dissolved/reacted in the water M4 - the carbonate was impure M5 - the temperature (of the gas) was lower than room temperature/5ºc Total 0

9 Answer Accept Reject Marks 6 (a) ( ) x =.5(0) x x 0 for mark (b) answer to (a) (c) answer to (b) x answer to (b) 5 for correct evaluation (= (0)) mark (d) M r of SO = 64 (e) answer to (c) x M r of SO (= 0.038(4)) Final answer must be to or more sig fig The wine is drinkable Ignore any explanations consequential on (d) Total 8

10 7 (a) B A D C Answer Notes Marks (b) Mixture Compound Mixture Total 7