OCR A Breath in chemistry 2020,, Chemistry As paper 1 2020

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OCR (Oxford Cambridge and RSA) is a leading UK awarding body, providing a wide range of qualifications to meet the needs of candidates of all ages and abilities. OCR qualifications include AS/A Levels... , Diplomas, GCSEs, Cambridge Nationals, Cambridge Technicals, Functional Skills, Key Skills, Entry Level qualifications, NVQs and vocational qualifications in areas such as IT, business, languages, teaching/training, administration and secretarial skills. It is also responsible for developing new specifications to meet national requirements and the needs of students and teachers. OCR is a not-for-profit organisation; any surplus made is invested back into the establishment to help towards the development of qualifications and support, which keep pace with the changing needs of today’s society. This mark scheme is published as an aid to teachers and students, to indicate the requirements of the examination. It shows the basis on which marks were awarded by examiners. It does not indicate the details of the discussions which took place at an examiners’ meeting before marking commenced. All examiners are instructed that alternative correct answers and unexpected approaches in candidates’ scripts must be given marks that fairly reflect the relevant knowledge and skills demonstrated. Mark schemes should be read in conjunction with the published question papers and the report on the examination. © OCR 2020 Annotations Annotation Meaning Correct response Incorrect response Omission mark Benefit of doubt given Contradiction Rounding error Error in number of significant figures Error carried forward Level 1 Level 2 Level 3 Benefit of doubt not given Noted but no credit given Ignore Blank page Abbreviations, annotations and conventions used in the detailed Mark Scheme (to include abbreviations and subject-specific conventions). Annotation Meaning DO NOT ALLOW Answers which are not worthy of credit IGNORE Statements which are irrelevant ALLOW Answers that can be accepted ( ) Words which are not essential to gain credit __ Underlined words must be present in answer to score a mark ECF Error carried forward AW Alternative wording ORA Or reverse argument SECTION A Question Answer Marks AO element Guidance 1 D 1 2.7 2 B 1 1.2 3 B 1 2.2 4 C 1 2.2 5 A 1 1.1 6 A 1 2.2 7 D 1 1.1 8 D 1 2.6 9 B 1 2.6 10 C 1 1.2 ALLOW 2 in the answer box 11 D 1 2.2 12 C 1 2.6 13 B 1 1.1 14 D 1 1.2 ALLOW 1 in the answer box 15 C 1 1.1 Total 15 5 SECTION B Question Answer Marks AO element Guidance 16 (a) (The mean/average mass) taking into account the relative abundancies of the isotopes  1 1.1 ALLOW sum of (isotopic mass  %abundance) sum of (isotopic mass  abundance) / total abundance DO NOT ALLOW average mass of the isotopes (i) 2+ – Mg x Br – x Br Mg with no (or 8) outer electrons AND 2  Br with ‘dot-and-cross’ outer octet  Correct charges  2 ALLOW 8 electrons in Mg2+ BUT ‘extra’ electron in Br– must match symbol for electrons in Mg2+ IGNORE inner shells and circles ALLOW 1 mark if both electron arrangements and charges are correct but only one Br is drawn. 1.2 ALLOW 2[Br–], 2[Br]– (brackets not required) 2.5 (ii) FIRST CHECK THE ANSWER ON ANSWER LINE If answer = 1.71  1022 award 3 marks ------------------------------------------------------------------------- n(MgBr2) = 1.74 = 0.00945….. mol  184.1 Moles of ions = 0.00945….  3 = 0.0283…. mol  Number of ions = 0.0283...  6.02  1023 = 1.71  1022  3SF required 3 2.23 ALLOW ECF Calculator answer = 9.451385117  10–3 ALLOW ECF from incorrect moles of ions. e.g. 0.00945 Common error 5.69  1021 no  3 2 marks [Show More]

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