2024 RVHS H2 Chemistry Prelim P4 (Ans)

2024 JC 2 H2 Chemistry 9729 Prelim Exam Paper 4 Suggested Solutions 1 (a) Titration number 1 2 Final burette reading /cm3 25.05 25.15 Initial burette reading /cm3 0.00 0.00 Volume of FA 2 (added) /cm3 25.05 25.15 (b) average volume of FA 2 used = 25.05 25.15 2 + = 25.10 cm3 volume of FA 2 used = ………25.10 cm3……….. (c) (i) amount of MnO4− used = 25.10 1000  0.010 = 2.51  10−4 mol amount of MnO4− = ……2.51  10−4 mol…….. (ii) amount of Fe2+ in 25.0 cm3 of FA 4 = 2.51  10−4  5 = 1.255  10−3 = 1.26  10−3 mol amount of Fe2+ = ……1.26  10−3 mol…….. (iii) amount of Fe2+ in 250 cm3 of FA 4 = 1.255  10−3  250 25.0 = 1.255  10−2 mol concentration of Fe2+ in FA 1 = 21.255 10 25.0 1000 − = 0.502 mol dm−3 concentration of Fe2+ = ……0.502 mol dm−3….... (iv) Mass of Fe(NO3)2 in 1 dm3 of FA 1 = 0.502  (55.8 + 2(14.0) + 6(16.0)) = 90.26 g Percentage by mass of Fe(NO3)2 = 90.26 125.4  100% = 72.0% percentage by mass of Fe(NO3)2 = ………72.0% …………..

River Valley High School Pg 2 of 9 JC 2 H2 Chemistry 9729 2024 Preliminary Examination Paper 4 Suggested Solutions (d) The titre value will be lower, because some of the Fe2+ in FA 1 would have been oxidised to Fe3+. Hence, less Fe2+ would be available for titration with KMnO4. Note: Sulfuric acid is added to prevent the oxidation of Fe2+ to Fe3+. (e) (i) In the titration of Fe 2+(aq) with MnO 4−(aq) in acidic medium, yellow Fe 3+(aq) and colourless Mn2+(aq) are formed. The end-point is reached when one excess drop of purple MnO4−(aq) is added, turning the solution from yellow to orange. (ii) There is no distinct/ visible/ significant colour change at the end-point. Hence, it would be impossible to know when the reaction is completed. OR H2O2 undergoes disproportionation/ decomposition, hence a higher volume of titrant will required. OR Fe2+/ Fe3+ present in the solution catalyses the decomposition of H2O2. (f) (i) % uncertainty in pipette = 0.06 25.0  100% = 0.240% % uncertainty in burette = 0.05 0.05 29.70 +  100% = 0.337% % uncertainty in measuring cylinder = 0.25 25.00  100% = 1.00% (ii) Burette Note: Measuring cylinder is not considered as FA 3 is added in excess.

River Valley High School Pg 3 of 9 JC 2 H2 Chemistry 9729 2024 Preliminary Examination Paper 4 Suggested Solutions 2 (a) Expt VFA 6 /cm3 𝑉H2O /cm3 Reaction time, t /s lg(VFA 6) lg (rate) 1 20.00 0.00 17.8 1.30 1.75 2 10.00 10.00 63.9 1.00 1.19 3 13.00 7.00 39.1 1.11 1.41 4 17.00 3.00 24.6 1.23 1.61 (b) (c) (i) Gradient of line = 1.63 1.20 1.235 1.005 − − = 1.87 (3 s.f.) m = 2 (nearest intege