Friday, September 28, 2007
helloo!!! okay, the answers for the chemistry practice paper:
Section B
B1(a)(i) Reagent: Aqueous Barium nitrate/lead(II)nitrate/calcium nitrate followed by dilute nitric acid.
Observation: White ppt formed insoluble in dilute nitric acid.
(ii) Reagent: Aqueous sodium hydroxide.
Observation: Green precipitate insoluble in excess aqueous NaOH.
(b) Anhydrous Iron(II)sulphate. Hydrated green crystals of iron(II)sulphate lost their water of crystallization and become white.
(c) X: Copper
Y: Sodium chloride
Z: Silver Chloride
(d) Magnesium is more reactive than copper and hence displace the copper ions from its solution to form copper metal which is reddish brown.
(e) Ag+(aq) + Cl-(aq) à AgCl(s)
B2(a) (i) Isotopes. and are both atoms of chlorine. They both have the same no. of protons which is 17 but different no. of neutrons. has 18 neutrons whereas has 20 neutrons thus different mass.
(ii) -. atom gains an electron to form - ion when reacting with metal.
(iii) Relative atomic mass of chlorine = (75/100)(35) + (25/100)(37)
= 35.5
(iv) this is supposed to be dot and cross diagram of Cl2O (chloride oxide)
(b)(i)
Name -Type of Structure -Electrical Conductivity(Molten State)
Silicon dioxide-Macromolecular -Does not conduct electricity
Calcium oxide -Ionic crystal lattice-Conduct electricity
(ii) SiO2 has macromolecular structure. Each Si atom joined to four other oxygen atom by strong covalent bond in a giant tetrahedral structure, all valence electrons used in bonding. Therefore, no free moving electrons to carry the electrical charges in any state. Ions in CaO held together by strong electrostatic forces of attraction in fixed position. In molten state, the ions are free to move hence able to conduct the electrical charges.
(iv) Calcium oxide is formed between Ca2+ and O2- ions whereas sodium chloride formed between Na+ and Cl- ions. The electrostatic forces of attraction between ions are stronger in CaO than NaCl because Ca2+ ion and O2-ion has a higher charge compared to sodium ion and chloride ion.
B3(a)(i) White solid: Magnesium oxide
Yellow solid: Sulphur
(ii) 2Mg + SO2 à 2MgO + S
(iii) MgO(s) + 2HCl(aq) à MgCl2(aq) + H2O(l)
(iv) SO2 2MgO 2MgCl2
No. of moles of SO2 = (4.0/32+2(16)) = 32.125 mol
No. of moles of MgCl2 = 0.120 mol
Max mass of MgCl2 formed = 0.120(24+2(35.5)) = 11.4g
(b)(i) No. of moles of CO2 = 18/24 = 0.75mol
(ii) CaCO3 CO2
Mass of CaCO3 = 0.750(40+12+3(16)) = 75g
(iii) % by mass of CaCO3 in sample = (75/100)(100) = 75%
Section C
C1(a) K+ and PO43-
(b)(i) Potassium hydroxide
(ii) H3PO4 + 3KOH à K3PO4 + 3H2O
(c) Titration mtd [1]
Correct steps [2]
Indicator usage + correct indicator [1]
(d)(i) Carbon dioxide
(ii)
Test: Bubble gas evolved into limewater
Observations: Limewater turned chalky/white ppt formed
(iii) Sodium is a reactive metal, it will react explosively with phosphoric acid which is dangerous.
C2(a) Beaker B
(b) Mg + Cu(NO3)2 à Mg(NO3)2 + Cu
Reddish brown/Pink solid deposited in the beaker. Blue solution slowly
turned colorless/fades to a lighter blue.
(c)(i) Beaker A and D
(ii) Aluminium forms a layer of protective oxide which is non porous and stick closely to surface of metal thus preventing aluminium from reacting with the solution.
(d) Calcium ion reacted with water to produce hydrogen gas.
C3(a) Correct steps and equations [3]
(b) Fe3O4
(c) Iron is a finite resource and hence need to be conserve.
(d)(i) Tin is unreactive and is used to prevent the food in the container from reacting with the steel alloy.
(ii) SnCl4
(iii) Small molecular structure. Weak Van Der Waals forces of attraction between tin(II)chloride molecules. Little energy needed to overcome these forces. Therefore m/pt low.sorry if its too messy... im bullshit with computers... heh, u guys will have to make do with this nonsense... so sorry hor.
`qin-
L ones are crappy~
10:26 PM
