RVHS 2026 Prelim Paper 4 Answer

2026 H2 Chemistry Prelim Paper 4 Student Answers 1 (a) (i) Titration number 1 2 Final burette reading/ cm3 24.25 44.15 Initial burette reading/ cm3 0.00 20.00 Volume of FA 2 used/ cm3 24.25 24.15 ✓ ✓ (ii) Average volume of FA 2 used = 24.25 24.15 2 + = 24.20 cm3 (b) Amount of S2O32− in 24.20 cm3 of FA 2 = 24.20 0.1001000  = 2.420  10−3 mol I3−  2S2O32− Amount of I3− reacted = 2.420  10−3 ÷ 2 = 1.210  10−3 mol = amount of I2 2Cu2+  I2 Amount of Cu2+ in 25.0 cm3 of FA 1 = 1.210  10−3  2 = 2.420  10−3 mol Amount of Cu in 1.00 dm3 of FA 1 = 2.420  10−3  1000 25.0 = 9.680  10−2 mol Mass of copper in 11.54 g of alloy = 9.680  10−2  63.5 = 6.147 g = 6.15 g Percentage by mass of copper in alloy = 6.147 100%11.54 = 53.3% (c) In (a)(i), amount of Cu2+ in 25.0 cm3 of FA 1 = 2.420  10−3 mol 2Cu2+ 4I− Amount of I− required to reduce Cu2+ in titration = 2.420  10−3  2 = 4.840  10−3 mol Amount of I− added in each titration = 40 0.2201000 = 0.00880 mol Ratio of amount of I− added: amount of I− required = 0.00880 : 4.840  10−3 = 1.82 : 1 (d) Not all the Cu 2+ will be reac ted and less I2 will be produced . Lower amount of I2 requires less amount of thiosulfate ions for titration, giving lower titre values.

2 (b) Expt Volume of FA 4 /cm3 Volume of H2O /cm3 t /s 1/t /s–1 lg(1/t) lg(Volume of FA 4) 1 50.00 0.00 16 0.0625 −1.20 1.70 2 15.00 35.00 62 0.0161 −1.79 1.18 3 27.00 23.00 33 0.0303 −1.52 1.43 4 39.00 11.00 22 0.0455 −1.34 1.59 (c) (i) Gradient = 1.24 ( 1.74) 1.67 1.23 − − − − = 1.14 m = 1

(ii) lg(volume of FA 4) = 1.31 From graph, when lg(volume of FA 4) = 1.31, lg(1/t) = −1.65 t = 10−1.65 = 45 s (iii) Expt Volume of FA 4 /cm3 Volume of H2O /cm3 Volume of FA 5 /cm3 t /s 2 15.00 35.00 5.0 62 5 15.00 30.00 10.0 61 Order of reaction with respect to H+ = 0 Comparing expt 2 and 5, time taken is approximately the same when V FA5 is doubled (while keeping V FA4 and V total the same). Rate of reaction is independent of [H+]. (iv) effect on t increases explanation water in beaker will decrease the concentration of reactants/ dilute the reaction mixture, decreasing rate of reaction, increasing t. (v) Experiment 1. Time taken for experiment 1 is shorter . Uncertainty due to human reaction time becomes more significant in experiment 1. OR Experiment 2. The obscuring of the print is gradual which makes it difficult to judge when to stop the stopwatch. Absolute uncertainty in experiment 2 is higher, giving rise to higher percentage uncertainty.

3 (a) Table 3.1 test observations FA 6 FA 7 (i) Add an equal volume of aqueous potassium iodide. FA 6 is a greyish-blue/ violet solution. [✓] No observable change. [✓] FA 7 is a yellow/ pale orange solution. [✓] FA 7 turns reddish brown/ red-brown/ brown solution. [✓] (ii) Add half a small spatula measure of zinc powder. Leave the mixture to stand for 3 min. Bubbles of gas observed. [✓] Solution remains greyish- blue/ violet. [✓] OR Colour of solution remains the same. Effervescence observed. [✓] Colourless, odourless gas evolved that gives a pop sound with a lighted splint. [✓] Gas is H2. [✓] Solution turns pale green/ colourless. [✓] OR Colourless solution formed. (iii) Add a few drops of aqueous silver nitrate. White ppt formed. [✓] White ppt formed. [✓] (iv) Add aqueous sodium hydroxide. Grey-green ppt formed, [✓] soluble in excess NaOH(aq) giving dark green solution. [✓] Red-brown ppt formed, insoluble in excess NaOH(aq). [✓] (b) Fill in the boxes below the identity of the metal cations in FA 6 and FA 7. FA 6 FA 7 cation Cr3+ Fe3+ (c) Anion: Cl− test observations To 1 cm 3 of FA 7 , add excess NaOH(aq). Filter the aqueous mixture. To the filtrate, add excess HNO3 (aq) followed by AgNO 3(aq), and then NH3(aq). White ppt soluble in excess NH 3(aq) to give a colourless solution. To 1 cm 3 of FA 6/ FA 7 , add acidified KMnO4 (aq) and warm the mixture. (Pungent and yellow−green) gas evolved that turns moist blue litmus red and bleaches it. Gas is Cl2.

To 1 cm 3 of FA 6/ FA 7 , add excess NH3(aq). Filter the aqueous mixture. To the filtrate, add excess HNO3 (aq) followed by AgNO 3(aq), and then NH3(aq). White ppt soluble in excess NH 3(aq) to give a colourless solution. (d) [Fe(H2O)6]3+(aq) + H2O(l) = [Fe(H2O)5(OH)]2+(aq) + H3O+(aq) 2H+(aq) + Zn(s) → H2(g) + Zn2+(aq) Zn(s) + 2Fe3+(aq) → Zn2+(aq) + 2Fe2+(aq) (e) Cr3+/FA 6, Zn2+, Fe3+/ FA 7 Zn reduces Fe3+. Zn does not reduce Cr3+. OR Fe3+ oxidises Zn. Cr3+ does not oxidise Zn.

4 (a) npropan-2-ol = 5.00 60.0 = 0.08333 mol nK2Cr2O7 = 0.08333 3 = 0.02778 mol mass of K2Cr2O7 = 294.2 × 0.02778 = 8.17 g (b) Procedure: 1. Weigh accurately about 5.00 g of propan-2-ol into a round bottom flask. 2. Add 8 drops of concentrated sulfuric acid to 5.00 g of propan-2-ol in the round bottom flask. 3. Weigh at least 8.17 g of potassium dichromate(VI) in a 250 cm3 beaker. 4. Using a 250 cm 3 measuring cylinder, add 100 cm3 of deionised water into the beaker to dissolve the potassium dichromate(VI). (Volume of water must be enough or in excess.) 5. Add this aqueous potassium dichromate(VI) solution slowly to the mixture in the round bottom flask. 6. Heat the mixture using the water bath for 20 minutes under reflux. 7. Separate the propanone from the reaction mixture using simple distillation by maintaining the temperature at 56.5 °C. I: Correct sequence (1 to 5): Reaction mixture in RB F includes, alcohol, conc acid, K2Cr2O7(aq). Allow “dissolves K 2Cr2O7 in DI in RBF first, then transfer 5 g of liquid propan-2-ol, with rinsing”. II: Heating under reflux for at least 20 minutes. III: Separation of propanone from reaction mixture using simple distillation at 56.5 °C. IV: Correct quantities of reagents: 5.00 g of propan-2-ol, 6 to 10 drops of concentrated sulfuric acid, at least 8.17 g of K2Cr2O7 in appropriate volume of water.

Diagram: • Heating reaction mixture using water bath/ heating mantle (with temp erature control) (A) • Bulb of thermometer at the opening of condenser and correct drawing of condenser showing coolant (water in and water out in correct positions) (B) • Sealed apparatus around the round bottomed flask and thermometer. No seal around collection flask. (C) • Propanone is distillate in diagram or in procedure (D) (c) Add sodium carbonate until no further effervescence is observed. OR Add solid base or metal until indicator shows that the reaction mixture is neutral or not acidic/ until some solid remains. OR Add alkali until indicator shows that the reaction mixture is neutral or not acidic.