2017 SAJC Preliminary Practical Examinations Marking Procedure Hierarchy to be used in calculating mean titres in question 1(a)(i):•value of 2 identical titres•average of titres within 0.05 cm3•average of titres within 0.1cm3, etc.Use the selected titres to determine the mean titre for the end-point. Note: For calculations, the principle of no double penalty (error carried forward) applies. For connecting parts, marking from point of first penalty onwards will be based on correct method only. Qn Skills assessed Marking Scheme Mark Mark Ref (MR) 1(a)(i) PDO Layout P1 Tabulates initial and final burette readings and volume added in the titration table. Table has correct headers and units. Tabulation may be vertical or horizontal; lines are not essential but there should be no absences of headers. Where units have not been included in the header, there should be the appropriate unit for each entry in the table. Do NOT award this mark if any final and initial burette readings are inverted or 50 is used as the initial burette reading. 1 1 PDO Record P2All the final/initial burette readings, for all accurate titres in the titration table, are recorded to the nearest 0.05 cm3. Treat all titres as "accurate" unless labelled 'rough' or first titre is recorded to a lower precision than subsequent titres. 1 2 MMO Quality M1Has at least two uncorrected titres for end-point within 0.10 cm3. Uncorrected titres refers to flawed calculated titres by the candidate. 1 3 A student's 'rough'/’trial' titre value can be considered by the examiner when selecting titre values for the mean titre calculation if the student has 'validated' this value either by ticking it or by using it in an expression in (a)(ii). (By doing either of these, the student has declared it to be no longer a 'rough’ / ‘trial' value).
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