Observation

This question was popular among the candidates. Majority of them attempted it, and their performance was above average.

In part (a), majority of the candidates stated the condition under which the reaction occurs. They also listed the species that were oxidized and reduced.

In part (b), majority of the candidates were able to list five steps in the treatment of water for town supply. However, only few of them were able to describe the steps involved in the treatment of water.

In part (c), majority of the candidates were able to write the electron configuration of the elements listed.

In part (d), majority of the candidates could not calculate the number of moles of hydrogen ions in 1 dm3 of a 0.1 mol dm-3 tetraoxosulphate (VI) acid solution.

The expected answers include:

(a) (i) Acidic medium/heat   
(ii)  I.C2O42-          
II.  MnO4-
(iii) MnO4- + 8H+ + 5e- → Mn2+ + 4H2O 
                                                                                                           
(b)  (i) Aeration
Coagulation/Flocculation/Addition of alum
Sedimentation
Filtration
Chlorination/disinfection/killing bacteria
Supplementation/addition of fluorine/iodine
pH measurement/regulation
                                                                                               
(ii) Aeration - exposure to air and sunlight

Coagulation - addition of alum brings about the coagulation of dirt particles in the water   

Sedimentation - the coagulated dirt particles sink to the bottom of the vessel       

Filtration - the use of sand bed (to bring about effective filtration of the water from foreign particles)

Chlorination    - addition of chlorine to the water (kills all inherent germs in the water making the water
chemically safe for drinking)

Supplementation - addition of other chemicals like fluorine and iodine supplements the usefulness of the water being purified to the human body   

pH measurement - addition of calculated amount of CaO or quicklime
                                   
(c)        (i)         12Q -  1s2 2s2 2p6 3s2 

                        17R -  1s2 2s2 2p6 3s2 3p5      

                        20S -  1s2 2s2 2p6 3s2 3p6 4s2 
(ii)  Chemical properties depend on the number of valence electrons. Q and S have the same number of valence electrons/2 electrons in the valence shell hence similar chemical properties. Q and R have different valence electrons, hence different chemical properties.

(iii) SR2    
                                                                                                                       
(d)       H2SO4(aq) → 2H+(aq) + SO42-(aq)
from the equation; 1 mole of acid   2 moles of H+ ions  
0.1 mole acid 0.1 x 2   
1
=    0.2 moles