2021 NYJC Prelim 9729 P4 (Ans)

Nanyang Junior College 2021 J2 H2 Chemistry Prelims Answers. 1 NYJC J2 H2 Chemistry Prelim Answers Paper 4 Answers 1 (a) (i) Dilution of FA 1 Final burette reading for FA 1 / cm3 Initial burette reading for FA 1 / cm3 Volume of FA 1 used / cm3 24.20 2 d.p. Titration Results Final burette reading for FA 4 / cm3 Initial burette reading for FA 4 / cm3 Volume of FA 4 used / cm3 19.15 19.05 2 d.p. 2 d.p. Correct header and units for both tables [1] Correct evaluation and precision for both tables [1] 24.25scaled mean titre = x mean titre volume diluted Scaled mean titre diff d r0.5 cm3 [2] or d r1.0 cm3 [1] (ii) Ave titre value = 19.10 (2 d.p.) Choice of titre values are consistent and average correctly evaluated [1] (b) (i) 2- 23 22 2 -3 SO 3 HO -3 -3 -3 -2 -3 22 19.10n reacted = 0.200 x = 3.82 x 10 mol1000 n in 25.0 cm reacted = n formed 1= x 3.82 x 10 mol2 = 1.91 x 10 mol [1] 1.91 x 10[H O ] = = 7.64 x 10 mol dm [1] 25.0 1000 FA 5 FA 5 I (ii) -2 22 22 -2 -3 22 250 24.207.64 x 10 x = [H O ] x ecf for [H O ] from 2(b)(i)1000 1000 2507.64 x 10 x 1000[H O ] = = 0.789 mol dm [1] 24.20 1000 FA 1 FA 5 FA 1

Nanyang Junior College 2021 J2 H2 Chemistry Prelims Answers. 2 (c) 2H2O2 J 2H2O + O2 2 2 2 3 O 3- 4 O 3- 4 O 1n evolved from 1 dm of = x 0.789 2 = 0.394 mol 0.3946n evolved from 1 cm of at r.t.p. = = 3 .946 x 10 mol [1] 1000 V evolved from 1 cm of at r.t.p. = 3.94 6 x 10 mol x 24000 c FA 1 FA 1 FA 1 3- 1 3 33 2 m mol = 9.47 cm [1] Hence 1 cm of gives 9.47 cm of O Vol st rength of is 9.47ŸFA 1 FA 1 (d) H2O2 in FA 5 is the limiting reagent while FA 3 is measured in excess. Using a measuring cylinder to measure FA 3 will not affect the amount of I2 produced hence will not affect the volume / accuracy of FA 4 used. (e) Impact: The oxidation of I– by H2O2 is incomplete when titration is carried out as the oxidation reaction is slow hence the concentration of H2O2 calculated is smaller than expected.[1] Evidence: The reaction mixture needs to stand for at least 7 minutes before titration. OR slow return of the blue black colour at end point suggest oxidation of I– still continues.[1] 2 (a) Temperature readings Time / min Temperature / oC Time / min Temperature / oC 0 29.3 7 ½ 36.2 1 29.3 8 36.4 2 29.2 8 ½ 36.4 3 29.2 9 36.4 4 –– 9 ½ 36.4 5 32.0 10 36.3 5 ½ 33.8 10 ½ 36.3 6 34.6 11 36.2 6 ½ 35.4 11½ 36.1 7 36.0 12 36.0 Correct heading and units [1] Record time to nearest half minute and temp to 1 d.p except at 4th min.[1]

Nanyang Junior College 2021 J2 H2 Chemistry Prelims Answers. 3 (b)

Nanyang Junior College 2021 J2 H2 Chemistry Prelims Answers. 4 Temperature change, 'T = oC [4] Correctly labelled axes and units, no odd scale used [1] Correctly plotted all points within ½ sq except at 4th min [1] Drew and extrapolated 2 best–fit lines to the 4th min, a