SAJC_2025 Dec Hol Revision Package (SDL)_Answers

ANSWERS TO 2024 DECEMBER HOLIDAY REVISION PACKAGE Topic 1: The Mole Concept and Redox Reaction 1. SRJC/PRELIMS 2011/P3/1(a), (b) (a) C H N O % by mass 50.7 7.0 19.8 22.5 Ar 12 1 14 16 Mole Ratio 4.225 7 1.414 1.406 Simplest Ratio 3 5 1 1 Let the molecular formula of acrylamide be C3nH5nNnOn. Mr of acrylamide = 71.0 = n (3 × 12.0 + 5 × 1.0 + 14.0 + 16.0) n = 1 Hence, the molecular formula of acrylamide is C3H5NO. Possible structures of acrylamide: Comments: Knowing the amide structure is the key to derive the possible structures. (b) Maximum mass of acrylamide the woman can take in per day = 0.5 × 10−3 × 50 = 0.025 g Mass of acrylamide in 1 kg of coffee = 1.2 × 10−7 × 71.0 = 8.520 × 10−6 g per kg of coffee Maximum mass of coffee the woman can drink per day = 0.025 ÷ (8.520 × 10−6) = 2934 kg Since coffee has the same density as water, maximum volume of coffee the woman can drink per day = 2934 ÷ 1 = 2934 dm3 C C C O NH2 H H H C O H N H C C H H H

2. RVHS/PRELIMS 2011/P3/4(b) (i) Element K Br O Mass ratio 23.4 47.8 28.8 Mole ratio 23.4 39.1 =0.598 47.8 79.9 =0.598 28.8 16 =1.8 Simplest ratio 1 1 1.8 0.598 =3.01= 3 Formula of white solid is KBrO3 (ii) No of moles of KBrO3= 0.835 167 = 0.00500 mol No of moles of PbCl2 = = 4.17 278 =0150 mol = (iii) 2e + Cl2 → 2Cl– X 3 3H2O + Br– → BrO3– + 6H+ + 6e [acidic medium] 6OH– + + Br– → BrO3– + 3H2O + 6e [alkaline medium] K+ ions are participating ions – Balance accordingly. 3Cl2 + KBr + 6KOH → 6KCl+ KBrO3 + 3H2O 3. H2S → S + 2H+ +2e Amount of BrOx− = (12.00/1000) x 0.300 = 0.0036 mol Amount of S = Amount of H2S = 0.289 / 32.1 = 0.009 mol Mole ratio of BrOx− : H2S = 1 : 2.5 = 2 : 5 1 H2S lost 2 e. 5 H2S lost 10 e. 10 e gained by 2 BrOx−. 1 BrOx− gains 5 e. Final oxidation state of Br in Br2 = 0 Initial oxidation state of Br in BrOx- is +5 5 + (−2)x = −1 x = 3   nKBrO3 nPbCl2 = 0.00500 0.0150   1 3

Criteria of success checklist:  provided clear statements for ALL working  presented intermediate answers to 4 s.f.  presented final answers to 3 s.f.  presented Mr to 1 d.p.  provided units, when applicable Things to note: • Conversion of units for volume: 1 dm3 = 1000 cm3 = 10−3 m3 1 cm3 = 10−3 dm3 = 10−6 m3 1 m3 = 1000 dm3 = 106 cm3 • In the calculation of empirical formula from experimental data, if the following figures are obtained for the number of moles, they are usually multiplied by a factor in order to get the correct simplest ratio. E,g 1.5 (multiply by 2), 1.33 (multiply by 3 ), 1.25 (multiply by 4), 1.2 (multiply by 5) • In balancing half equations, 1. balance the atom that is not O or H first. 2. Balance O by adding H2O 3. Balance H by adding H+ 4. Balance charge by adding electrons Conti