Punggol Prelim P2 Ans

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 1 Section A ( 70 marks) 1 Suggested solution accept reject (a) ZnCl2 [1] (b) PbO [1] (c) V2O5 [1] (d) (i) NH4Cl [1] (ii) NaOH + NH4Cl → NaCl + H2O + NH3 [1] 2 Suggested solution accept (a) Isotopes are atoms of the same element with the same number of protons but different number of neutrons. [1] (b) (i) particle number of protons number of electrons number of neutrons G1 16 16 16 G2 16 18 18 G3 16 16 20 [2]: 5 correct [1]: 1 to 4 correct (ii) Nuclide notation of atom G2: S16ଷସ [1]: chemical symbol, proton number and mass number. 3 Suggested solution (a) (i) gases time/s relative molecular mass carbon monoxide, CO 119 28 chlorine, Cl2 190 71 methane, CH4 90 16 oxygen, O2 127 32 [1] All correct (ii) Time taken = k x Mr For CO, k = 119 / 28 = 4.25

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 2 For Cl2, k = 190 / 71 = 2.68 For CH4, k = 90 / 16 = 5.63 For O2, k = 127 / 32 = 3.97 The measurements do not support the conclusion. [1] The constant, k, is not the same for the gases. [1] Award 2nd mark if student compares at least two k values. (b) Accept any time between 127 seconds and 190 seconds. [1] Sulfur dioxide is soluble in water so the water level rises faster than pre- dicted. [1] 4 Suggested solution (a) Cu2O is oxidised to CuSO4 because the oxidation state of copper increases from +1 in Cu2O to +2 in CuSO4. [1] Cu2O is reduced to Cu because the oxidation state of copper decreases from +1 in Cu2O to 0 in Cu. [1] This is disproportionation as Cu2O is oxidised and reduced simultaneously. [1] (b) (i) Element Cu O % by Mass 84.1 15.9 Number of moles/ mol ଼ସ.ଵ ଺ସ = 1.3141 ଵହ.ଽ ଵ଺ = 0.99375 Mole ratio (divide by the smallest number) ଵ.ଷଵସଵ ଴.ଽଽଷ଻ହ = 1.3224 ଴.ଽଽଷ଻ହ ଴.ଽଽଷ଻ହ = 1 X3 X3 Simplest ratio 4 3 Empirical formula is Cu4O3 [1] Working + Empirical formula Let the molecular formula of paramelaconite be (Cu4O3)n. (4 x 64 + 3 x 16)n = 304 n = 1 Working to prove that the molecular formula of paramelaconite is Cu4O3. [1] (ii) Cu4O3 + 2H2SO4 → Cu2O + 2CuSO4 + 2H2O [1] 5 Suggested solution (a) Atoms of the different metals have different sizes. The regular layers of atoms are disrupted. [1] The layers of atoms cannot slide over each other easily when a force is applied. [1]

PSS Science Chemistry PSS 2024 Mark Scheme for CHEMISTRY -6092 : Sec 4 : Prelim 3 (b) (i) Insert a lighted splint into the gas. Lighted splint extinguishes with a ‘pop’ sound. [1] (ii) Chromium sulfate is insoluble in water. [1] A layer of insoluble chromium sulfate will form over the surface of chromium metal and prevent the reactants from reacting further. [1] (iii) Add aqueous chromium nitrate to sulfuric acid. Filter to collect chromium sulfate precipitate. Wash the