SAJC H1 CHEM P1 Workout solutions

1 2015 H1 Chemistry Preliminary Examination Paper 1 Workout Solutions 1 Trichloroisocyanuric acid is used c ommonly in swimmi ng pools as a disinfectant. The recommended concentration level of the acid is 1.50 mg per litre. Trichloroisocyanuric acid How many moles of chlorine atoms are present in a 2.50 x 106 litre Olympic-sized pool? Working: 3 atoms in 1 molecule of Trichloroisocyanuric acid No of moles of Trichloroisocyanuric acid in the pool = 14x335.5x316x3 12x3 10 x 1.50 x 10 x 2.50 36  =16.1 No. of moles of chlorine atoms = 3 x 16.1 = 48.3 (Answer: B) 2 Carbon sulfide, CS2 is a volatile flammable liquid used in the manufacture of cellophane. It combusts to form CO2 and SO2. What is the volume of gases produced after the complete combustion of 30 cm3 of CS2? Working: CS2 (g) + 3O2 (g)  CO2 (g) + 2SO2 (g) Volume of gas produced = 3 x 30 = 90 cm3 (Answer: C)

2 3 The active ingredient in antacids is calcium carbonate. To assess the percentage content of calcium carbonate in a tablet, a 0.50 g tablet was dissolved in 30 cm 3 of 0.150 mol dm-3 hydrochloric acid. The resultant solution required 21.30 cm3 of 0.100 mol dm-3 sodium hydroxide for complete reaction. What is the percentage mass of calcium carbonate in the antacid tablet? Working: NaOH + HCl  NaCl + H2O 2HCl + CaCO3  CaCl2 + H2O + CO2 No. of moles of NaOH = 3-10 x 2.13 0.1 x 1000 21.30  No. of moles of HCl reacted with CaCO3 = 33- 2.37x1010 x 2.13 - 0.150 x 1000 30.00  No. of moles of CaCO3 = -33 10 x 1.185 22.37x10  Mass of CaCO3 = 1.185 x 10-3 x (40 + 12 + 16x3) = 0.1185g % mass of CaCO3 = 23.7% 100 x 0.500 0.1185  (Answer: A) 4 720 cm 3 of H 2S gas is passed through 40 cm 3 of 0.500 mol dm -3 HNO3. Yellow precipitate of S was produced after the complete reaction as shown in the following half equation. Which of the following could be the nitrogen-containing product? (Assume that all volumes are measured at room temperature and pressure conditions) Working: No. of moles of H2S = 0.03 24000 720  No. of moles of HNO3 = 0.02 0.500x 1000 40  Mole ratio of H2S: HNO3 = 3: 2 1 mole of H2S lost 2 electrons  3 moles of H2S lost 6 electrons 2 moles of HNO3 gains 6 electrons  1 mole of HNO3 gain 3 electrons Original oxidation state of HNO3: (+1) + x + 3(-2) = 0 x = +5 Since HNO3 gains 3 electrons, new oxidation state of N = +2 (Answer: B)

3 Compound Oxidation State of N A N2 0 B NO +2 C NO2 +4 D NH3 -3 5 Barium carbonate is an insoluble salt composed of barium and carbonate ions . When the two ions are passed through an electric field, it was observed that the barium ions are deflected by an angle of −5.8o. What would be the angle of defection for the carbonate ions? Working: Angle of deflection = )mass charge( x k −5.8 = )137 2( x k  k = −397.3 Angle of deflection = )16x3 12 2-( x (-397.3  = 13.2o (Answer: C) 6 The following data shows the successive ionisation energies of two elements. Elements 1st 2nd 3rd 4th 5th 6th 7th 8th X 1060 1900 2920 4960 6280 21200 25430 29872 Y 1251 2298 3822 5159 6542 9362 11018 33604 Which of the following could be the compounds formed between X and Y? Largest increase in IE for X: From 5th to 6th IE  X is from Group 5 Largest increase in IE for Y: From 7th to 8th IE  Y is from Group 7 Possible compounds formed: XY3 or XY5 (Answer: C) 7 Which of the two species when mixed together will give the most exothermic interactions? Type of solute- solvent interactions A dichloromethane and propane Pd – pd vs id – id B tetrachloromethane and ethanol Id – id vs hydrogen bonding C propanal and propane Pd – pd vs id-id D propanal and dichloromethane Pd – pd vs pd – pd (Answer: D)

4 8 Which of the following pairs contain one simple covalent and one giant covalent compound? Type of structures A aluminium fluoride and silicon dioxide Giant ionic compound & giant covalent compound B sulfur dioxide and silicon Simple covalent compound & giant covalent compound C boron fluoride and silicon tetrachloride Both are simple covalent compound D sodium oxide and graphite Giant ionic compound & giant covalent compound Answer: B 9 Humulene can be extracted from carnation flowers. Humulene How many sp2 – sp3 hybrid orbitals overlap are present in humulene? Answer: C 10 Which of the following sequence gives the lattice energies in order of increasing magnitude? Working: Order of cationic charge: Ba2+ and Mg2+ > Na+ and K+ Order of anioic charge: S2− and O2− > Br− and I−  L.E of BaS and MgO > L.E of NaBr and KI Order of size: Ba2+ > Mg2+ and S2− > O2−  L.E of MgO > L.E of BaS Order of size: K+ > Na+ and I− > Br−  L.E of NaBr > KI  Order: KI < NaBr < BaS < MgO (Answer: C)

5 11 The use of the Data Booklet is required for the following question. The combustion of propyne is shown below. What is the standard enthalpy change of combustion of propyne? Working: Bond breaking = 4(C-H) +(C-C) + (C≡C) + 4(O=O) = 4(410) + (350) + (840) + 4(496) = 4814 Bond forming = 6(C=O) + 4(O-H) = 6(740) + 4(460) = 6280 H= Bond breaking – bond forming = 4814 – 6280 = -1466 kJ mol-1 (Answer: A) 12 Which of the following is always an endothermic process? Answer: D A Enthalpy change of combustion Always exothermic B Enthalpy change of formation Can be exothermic or endothermic C Enthalpy change of neutralisation Can be exothermic or endothermic. SA-SB: ≈ −57.3 kJ mol- D Lattice dissociation energy Ionic bonds are broken so always endothermic 13 Which of the following shows the correct fraction of molecules with energy greater than activation energy, Ea, at the respective temperatures? Answer: B

6 14 The rate of the reaction between M and L is shown below. Rate = k [M][L]2 When the concentration of M is doubled, the reaction proceeded at half of its original rate. What was the corresponding change made to the concentration of L? Working: When concentration of M is doubled  Rate will double. To cause it to proceed at half its original rate  L must be decreased by half. (Answer: C) 15 There are 39 known isotopes of radon (Rn) from 193Rn to 231Rn. The most stable isotope is 222Rn with a half-life of 3.8 days. How long will it take for 222Rn to decrease to one-eighth of its original amount? Working: 2 1  4 1  8 1 3 half-lives = 3.8 x 3 = 11.4 days (Answer: B) 16 A mixture of nitrogen, hydrogen and ammonia were reacting at dynamic equilibrium via the Haber Process. When 0.5 mole of hydrogen chloride was passed through the system, which of the following will occur? Working: N2 + 3H2  2NH3 When HCl is added to the system, it will react with NH 3. Position of equilibrium shifts to the right to produce more NH3 to replenish the loss. Answer: C 17 Which of the following solutions when mixed together will form a buffer solution? Answer: D Type of solution A 0.5 mol of ethanoic acid and 0.5 mol of sodium hydroxide Basic salt: CH3CO2 −Na+ B 0.5 mol of sulfuric acid and 1.0 mol of ammonia Acidic salt: (NH4)2SO4 C 1.0 mol of ethanoic acid and 2.0 of sodium hydroxide Strong base: NaOH D 1.0 mol of sulfuric acid and 3.0 mol of ammonia Buffer: (NH4)2SO4 and NH3

7 18 Which of the following graph shows the correct pH trend of oxides of Period 3? Working: Compound Na2O MgO Al2O3 SiO2 P4O10 SO2 pH 14 9 7 7 3 2 Answer: A 19 M is an element in Period 3. Its oxide does not dissolve readily in water but spontaneously in aqueous sodium hydroxide. The molten oxide of M has a high electrical conductivity. Which of the following statement about M is not true? Working: M is Aluminium (Answer: A) A M exist as a giant covalent structure. Al exist as a giant metallic structure. B The oxide of M dissolves in aqueous hydrochloric acid. Al2O3 is amphoteric. C The aqueous solution of the