2020 RI Prelim P4 Ans (students)

1 © Raffles Institution 2020 9729/04/S/20 2020 RI H2 Chemistry Prelim Paper 4 – Suggested Solutions 1(a)(i) Table 1.1 final burette reading / cm3 25.40 35.40 Note: (also applies to Table 1.2)  Record burette readings to 2 d.p.  Get consistent results (i.e. at least two titre values which are within 0.10 cm3 of each other). initial burette reading / cm3 0.00 10.00 volume of FA 2 used / cm3 25.40 25.40 Comments:  Generally well done. 1(a)(ii) V1 = 25.40 + 25.40 2 = 25.40 cm3 Note:  Average volume, V1, should be calculated using consistent titres (within 0.10 cm3 of each other) obtained in Table 1.1.  Working must be shown.  Average volume to 2 d.p. Comments:  Generally well done. 1(b)(i) Amount of KMnO4 used = 25.40 1000  0.0200 = 0.000508 mol From equation 2, mole ratio of MnO4− : Fe2+ = 1 : 5 Amount of Fe2+ = 0.000508  5 = 0.00254 mol [Fe2+] = 0.00254 10  1000 = 0.254 mol dm−3 Note: For all questions involving calculations,  statements and working should be clearly shown.  all final answers to 3 s.f. Comments:  Generally well done. 1(b)(ii) Concentration of hydrated halotrichite in FA 1 = 22.26 100  1000 = 222.6 g dm−3 Since each formula unit of the hydrated halotrichite contains only one Fe2+, from (b)(i), concentration of hydrated halotrichite in FA 1 = concentration of Fe2+ = 0.254 mol dm−3 Relative formula mass of hydrated halotrichite = 222.6 0.254 = 876 Comments:  Some students did not read the question carefully – FA 1 is prepared by dissolving 22.26 g of hydrated halotrichite in only 100 cm3 of water.  A few students incorrectly gave units for the Mr of the hydrated halotrichite. Note that o Relative formula mass (Mr) and relative atomic mass (Ar) have no units o Units for molar mass is g mol−1

2 © Raffles Institution 2020 9729/04/S/20 1(c) Table 1.2 Volume of FA 3 added / cm3 0.00 6.00 12.00 18.00 final burette reading for FA 2 / cm3 19.30 33.40 42.50 initial burette reading for FA 2 / cm3 0.00 20.00 35.00 volume of FA 2 used / cm3 25.40 19.30 13.40 7.50 Comments:  Generally well done.  Some students did not follow the question and failed to select volumes of FA 3 which differ by at least 6.00 cm3. 1(d) Comments:  Generally well done.  The scale chosen for x-axis should allow room for extrapolation to obtain V2.  Avoid odd scales, e.g. one large square for 4 cm 3 or 6 cm 3. Such odd scales will be penalised as it makes the plotting and/or reading of coordinates difficult. 1(e)(i) From graph, V2 = 25.50 cm3 Comments:  Generally well done.  To obtain V2 (volume of FA 3 required to react completely with 10.0 cm 3 of FA 1 with no FA 2 added), the extrapolation of graph to obtain the x-intercept must be clearly shown. Note:  The x -axis and y -axis should be correctly labelled with the correct units.  Appropr