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Friday, December 26, 2008

GCE O Level Pure Chem Answers 1998 TYS

Posted by Sg Student resources

GCE ‘O’ Level Pure Chemistry (5070) November 1998

 

Paper 1 (5070/01)

 

1

2

3

4

5

6

7

8

9

10

B

D

D

A

A

B

D

D

A

C

 

11

12

13

14

15

16

17

18

19

20

B

C

D

D

C

B

C

B

C

D

 

21

22

23

24

25

26

27

28

29

30

C

B

B

B

A

C

B

B

B

D

 

31

32

33

34

35

36

37

38

39

40

A

D

C

B

B

C

B

C

C

B

 

Question

Explanations

 

6

Xenon is incorrect as it does not have the same number of electrons as Krypton. However, it has the same number of outer electrons. The answer is Rubidium (Group I) as it has 37 electrons and loses 1 electron to form Rb+ which gives 36 electrons.

14

A solution of an ionic compound contains H+ and OH- from water as well as the constituent ions of the compound.

19

Potassium manganate (VII) is the only oxidizing agent present in the options which can oxidize iron (II) ions to iron (III) ions.

21

Calcium carbonate when added in excess would neutralize nitric acid without leaving an alkaline solution.

34

Graphite, consists of only one type of atom i.e. carbon atoms and therefore is an element and not a compound.

 

Paper 2 (5070/02)


Section A and B

 

Qn

Answers

A1

(a)  Cl

 

(b)  I, Cl, H, O, N, Br (any one of these)

 

(c)  F

 

(d)  Na, K (any of the Group I metal)

 

(e)  O

 

(f)  Fe

 

{note: no marks will be awarded for writing answer in words  instead of symbols}

 

A2

(ai)  tungsten and aluminium

 

(aii) they conduct electricity when solid and have high melting point

 

(b)  Wax melts over a range of temperature (35-50oC)

       {common mistake: mixture has a low melting point}

 

(ci)  sodium chloride

 

(cii)A substance containing 2 or more elements joined together by a chemical reaction.

 

(d)  At 659oC, aluminium is a solid. The particles are closely packed in an orderly arrangement. The particles can only vibrate about their fixed position. At 661oC, aluminium became a liquid. The particles are now in a disorderly arrangement and they can move throughout the liquid.

 

(e)  Add water to the mixture. Sodium chloride dissolves but not sulphur. Filter the mixture. The residue is sulphur and the filtrate is sodium chloride solution. Heat the sodium chloride solution in an evaporating dish until all the water has evaporated. Sodium chloride is left in the dish.

 

A3

(a)   A substance containing carbon and hydrogen only.

       {common mistake: omitting the word ‘only’}

 

(b)   Fraction D

 

(ci)  X is C4H8

 

(cii)        H H H  H                  H H  H H

               |    |   |    |                    |   |    |   |

          H-C-C-C=C-H          H-C-C=C-C-H

               |    |                             |            |

              H  H                           H          H

 

(ciii) The hydrocarbon X can be polymerized because it contains a carbon-carbon double bond.

 

A4

(a)   2H2 + O2 ® 2H2O

 

(b)  Redox reaction is a reaction in which oxidation and reduction occurs at the same time. In this reaction, one substance gains oxygen while the other substance loses it oxygen.

       {must mention the second sentence to earn the full marks}

 

(c)   This reaction is exothermic because ∆H is negative.

 

(d)  Burning of hydrocarbon is likely to lead to the formation of carbon monoxide which is a pollutant. Burning of hydrogen will only form water which is not a pollutant.

 

(e)   No. of moles of hydrogen gas = 50/2 = 25 moles

        Volume of hydrogen gas = 25 x 24 = 600 dm3

       {Note: hydrogen gas in H2 not H}

 

A5

(ai)  Beaker B

 

(aii) 2 AgNO3 + Cu ® Ag + Cu(NO3)2

        {Common mistake: writing CuNO3}

(b)  Beaker A and D {because Zn is more reactive than Ag and Cu. A more reactive metal will displace a less reactive metal from its salts}

 

(c)  Aluminium has a layer of oxide coated on it. The oxide layer protects the aluminium from reacting with other substances.

 

A6

(a) 

particle

charge

mass

Proton

+1

1

Neutron

0

1

Electron

-1

1/2000

       {Common mistake: never include ‘+’ sign for the charge of proton}

 

(b) Because they have the same number of protons but different number of neutrons

 

(c)   The relative molecular mass of a Tritium molecule, T2 is 6.

 

       Explanation: 1 Tritium atom has 1 proton, 1 electron and 2 neutrons. Hence, the relative atomic mass of 1 Tritium atom is 3 and the relative molecular mass of a Tritium molecule, T2 is 6.

 

        {Common mistake: giving relative atomic mass (which is 3) as the answer}

 

(di)   CaH2

 

-

 
(dii) 


 


{Note: Hydride ion is H-. Therefore, hydride ion is formed when hydrogen takes in 1 more electron}

 

A7

(a)  1. The impure copper should be connected to the positive terminal of the battery and the pure copper – the negative terminal.

        2. The electrolyte used should be copper (II) sulphate solution and not NiSO4.

 

(b)   Cu(s) – 2e ® Cu2+ (aq)

        {Common mistake: failure to include state symbols in equation}

 

 

Section B

B8

(a)   Na+ and PO43-

 

(bi)  sodium hydroxide

 

(bii) H3PO4 + 3 NaOH ® Na3PO4 + 3H2O

 

(c)  Titration method is used. Fill a burette with phosphoric acid. Pipette 25cm3 of sodium hydroxide into a conical flask. Add in 2 drops of the given indicator. Add in phosphoric acid from the burette until the conical flask shows a colour change. Note the volume of phosphoric acid used. Repeat the experiment using 25cm3 of sodium hydroxide and the same volume of phosphoric acid with no indicator added. The solution obtained is pure sodium phosphate. Pour some of the solution into an evaporating dish. Warm the solution until it is concentrated. Allow the concentrated solution to cool. Crystals of sodium phosphate is formed. Filter to remove the crystals and dry between sheets of filter papers.

       {Common mistake: (1) fail to mention the need to repeat the titration WITHOUT an indicator (2) mention ‘evaporation to dryness’ or vague answers such as ‘heating the solution’ without making a clear point whether it was to dryness or saturation}

 

(di)  Carbon dioxide

 

(dii) Bubble the gas into limewater. A white precipitate is formed.

 

(ei)  Ca3(PO4)2

 

(eii) Calcium phosphate provides the plant with phosphorus.

 

B9

(ai)   Ethanoic acid               H  O

                                              |    ||

                                         H-C-C-O-H

                                              |

                                             H

          {Common mistake: missing bond between O and H}

 

(aii)  The acid is formed when ethanol is oxidized by bacteria from air or by adding acidified potassium dichromate (VI) which is an oxidizing agent.

 

(bi)  Phosphoric acid as catalyst, temperature of 300oC and 65 atmospheric pressure

 

(bii)   Given 72 dm3 of ethene

          No. of moles of ethanol/no. of moles of ethene = 1/1

          No. of moles of ethene = volume/molar volume = 72/24 = 3 moles

          No. of moles of ethanol = 3 moles

          Mass of ethanol = 3 x molar mass = 3 x (12x2+1x5+16+1) = 138g

 

(c)    The process is very slow./ This process only produces a dilute solution of ethanol (10-12%).

 

(d)    Ethanol is used as a solvent in perfumes and paints, as a fuel for motor vehicles, in the manufacture of ethanoic acid.

          {Common mistake: vague answers such as ‘solvent’ (neglecting ‘in product’ such as perfumes) and ‘fuel’ (neglecting ‘for motor vehicles’) were insufficient to score}

 

B10

(ai)     Lithium floats on water and dissolves rapidly. Effervescence is seen.

           {Note: question is looking for observations and not inferences}

 

(aii)    Add a piece of red litmus paper. The paper will turn blue.

 

(bi)     The rate of reaction decreases (or the reaction slows down with time)

           {Common mistake: rate becomes ‘constant’}

 

(bii)    Because lithium is used up completely

 

(c)      8 seconds {Common mistake: 33 seconds}

 

(di)     340 cm3

 

(dii)    Volume of H2 produced in experiment 2 = 170 cm3

           Volume of H2 produced in experiment 3 = 340 cm3

 

(diii) Rate of reaction for experiment 2 is slower than experiment 1. This is because 0.1g of lithium has a smaller surface area than 0.2g of lithium. Rate of reaction for experiment 3 is faster than experiment 1. This is because of higher temperature.

 

B11

(a)     Place a moist blue litmus paper into the gas jar. The gas will turn the blue litmus paper red and then bleached.

 

(b)    A white solid is formed. (because NaCl is formed and NaCl is a white solid)

         {Note: question is looking for observations and not inferences}

        

(c)            2 Na + Cl2 ® 2 NaCl

{Note: Chlorine is Cl2 not Cl!}

 

(d)    


(ei)    Sodium chloride is an ionic compound. The ionic bonds holding the ions are very strong. Lots of energy is needed to break the bonds. Therefore, sodium chloride has a high melting point.

 

(eii)    When molten, the sodium ions and chloride ions can move about freely to carry current.

          {Common mistake: moving ‘electrons’ instead of ‘ions’}

 

 

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