Majority of the candidates gave correct shape of s-orbital and p-orbital but many of the candidates could not identify the correct ion with the composition of the particles stated in the question.
A fair attempt was made at identifying ionic and covalent bonds in the given oxides based on their properties but quite a large number of the candidates could not explain the formation of the covalent bonding in Z2O.
A fair understanding of equilibrium reaction was displayed by majority of the responding candidates except for the features of an equilibrium reaction that was poorly tackled.
Majority of the candidates were able to calculate the empirical and molecular formulae.  However, few candidates lost marks because they cannot resolve the fractions in the mole ratio of the atoms into whole number e.g. 1 : 1.5 : 1 should have been resolved to 2 : 3 : 2 however, majority of the candidates approximated it to 1 : 2 : 1 which was wrong.
The expected answers are:
3(a)      (i)         I.          s-orbital = spherical shaped     accept drawing O
                        II.        p-orbital – dumb-bell shaped accept drawing shape of figure 8
(ii)        Helium nucleus/ion/He2+/alpha particle.

     (b)   (i)         X ionic/electrostatic forces/electrovalent/coulombic forces
                        Y covalent
                        Z  covalent
(ii)        Two Z atoms donate one electron each while an oxygen atom donates two electrons to form two shared pair of electrons.

1. I.   Y2O is hydrogen bonding/dipole-dipole. 

           II.   Z2O is vander waal forces
     (c)   (i)      All the species in the equilibrium are gaseous/in same physical state/phase
 (ii)     I.      Equilibrium position is unaffected.  There is equal number of gaseous    molecules/moles on either side of the equilibrium.

1. Rate increases.  Increasing the pressure decreases volume/increase the concentration

(iii)         The forward reaction proceeds at the same rate as reverse reaction

The concentration of reactants and products are constant

   Equilibrium can only be achieved at a given temperature

Equilibrium can only be achieved in a closed vessel if there is a gaseous  component.

1. Na                               S                                             O
   29.1                          40.5                                      30.4
   23.0                          32.0                                      16.0    
   1.26                         1.26                                   1.90
   
                           Empirical formula  = Na2S2O3
                  (ii)     (Na2S2O3)n = 158
                           (23 x 2 + 32 x 2 + 16 x 3)n = 158
                           n  = 158  = 1                                                                                                                                                                             158
   Molecular formula = Na2S2O3