Chemistry 2 Nov/Dec 2011

Chemistry Paper 2,Nov/Dec 2011

Questions:		1	2	3	4	7	8		Main

General Comments
Weakness/Remedies
Strength

Question8

(a) Describe briefly how each of the following aqueous solutions could be identified in the laboratory:

(i) Ammonium trioxocarbonate (IV);

(ii)Ammonium chloride.

(b) Arrange the following compounds in order of increasing boiling point and give reasons for your answer:
CS2, NaF and CO2.

c.List two gases each that are:

(i) acidic;

(ii) highly soluble in water;

(iii) oxidized by acidified KMnO4(aq).

  (d) In a tabular form, compare the elements silicon and sulphur under the following properties:
            (i)         metallic character;
            (ii)        physical state;
            (iii)      conduction of electricity.

   (e) A cuboid piece of sodium metal measures 3 cm x 4 cm x 10 cm. If the density of sodium is
            0.971 g cm-3, calculate the number of atoms in the sodium metal.
            [ Na = 23; Avogadro constant = 6.02 x 1023 mol_1 ]

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OBSERVATION

Quiet a large number of the candidates could not describe how to identify the given solutions which required application of practical works.
The reasons for increasing boiling points in the given compounds were not correctly stated by most candidates.
A fair attempt was made at identifying gases based on their properties.
A good attempt was also made at comparing silicon and sulphur based on their properties.
This question was well attempted by majority of the responding candidates.

The expected answers are:

Test	Observation	Inference
Aqueous solution of (NH4)2 CO3 + dil HCL/HNO3/H2SO4	Brisk effervescene. Colourless gas turns lime water milky	CO2 from CO32-
NH4CL + AgNO2(aq) + dil HNO3	White precipitate insoluble in excess HNO3	Cl- present

To each of the solutions add dil HCl/HNO3/H2SO4
Brisk effervescence; (NH4)2CO3 confirmed
No reaction; CL- confirmed.
And is therefore ammonium chloride.

(b)        CO2; CS2, NaF
            Increasing order of boiling point
CO2; weak van der Waal’s forces due to large size
            NaF; ionic forces
            Ionic forces stronger than van der Waal’s forces

(c)        (i)         NO2, CO2, HCl, Cl2, SO2, SO3, etc.
            (ii)        NH3, HCl
            (iii)       SO2 H2S

Silicon	Sulphur
A metalloid	Non-metal
A Solid	A solid
Semi conductor/conducts	Non-conductor of electricity

(They must correspond to score)

(e)        Volume of sodium      =          1 x b x h
                                                =          3 x 4 x 10
                                                =          120 cm3
            Mass = Density x Volume
                     = 0.971 x 120 = 116.52g
            23g of Na contains 6.02 x 1023 atoms
            116.52g of Na = 116.52 x 6.02 x 1023
                                                23
                                    =   3.05 x 1024 atoms