Chemistry Paper 2,Nov/Dec 2010  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 1

(a) Define each of the following terms:
              (i)         atomic number;
              (ii)        isotopy. [4 marks]

(b) Two naturally occurring isotopes 3517R and 3717R have relative abundances of 75.5% and 24.5% respectively.
        (i) State the subatomic particles and the corresponding number present in each of the isotopes.

        (i)       Determine the:
                  I.           relative atomic mass of element R;
                  II.         number of molecules present in 17.75 g of R.

[ Avogadro's constant = 6.02 x 1023 particles mol"] [12 marks]

(c) (i) Define the term natural radioactivity.
(ii) State the difference between natural radioactivity and artificial radioactivity
(iii) Copy and complete the nuclear reaction:

23592U + 10n →x92U →9236Kr + 14156Ba + y10n +Energy

(d) Explain why ethanol is completely miscible with water



The question was attempted by most candidates and the performance was good.

In part (a)(i) and (ii), candidates correctly defined atomic number and isotopy thus:
- atomic number is the number of protons (in the nucleus) of an atom of an element/the number of electrons in a neutral atom.
- isotopy is a phenomenon whereby atoms of an element exhibit different mass numbers but have the same atomic number/the same number of protons but different number of neutrons.

In part bli], candidates correctly stated the sub atomic particles and the corresponding number present in the two isotopes as follows:

                Proton         Electron          Neutron
3517R          17                  17                   18
3517R          17                   17                   20

In (b)(iiL I and II, candidates correctly calculated the relative atomic mass of the element but most of them could not infer that the element is diatomic and hence lost mark in II. The expected calculations from candidates were as follows:
(I)          ReI. atomic mass of R = (75.5 x 35 + (24.5 x 37)
                                   =35.49 or = 35.5
(II)         Molar mass of R2 = 2 x 35.5
                                      = 71 gmol-1
                   71g → 602 x 1023
                   No. of molecules of R2
                   = 6.02 X 1023 x 17.75
                   1.51 X 1023 molecules

In part (c)(i),(ii), (Hi) and (Iv), candidates correctly defined natural radioactivity, stated the differences between natural and artificial radioactivity, copied, and completed the given nuclear reaction and stated the type of reaction exhibited by the reaction thus:

   (i)           It is spontaneous disintegration/decay of a nucleus to produce radiations/particles (and energy)

Natural Radioactivity

Artificial Radioactivity

New nuclei exist naturally

Most of the nuclei do not exist



It is a spontaneous process


23592U + 10n →23692U →9236Kr + 14156Ba + 310n +Energy

(iv)               Fission

In part(d), only few candidates knew that ethanol is completely miscible with water because the two compounds contain hydroxyl group (OH)/both are polar which allows for (intermolecular) hydrogen bonding.

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