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 Chemistry Paper 3,Nov/Dec 2008
 Questions: 1 2 3 Main
Weakness/Remedies
Strength

Question 1

The following results were recorded by a student when 25.00cm3 portions of a solution of an impure potassium trioxocarbonate (IV) were titrated against 0.100 moldm -3 tetraoxosulphate (VI) acid.

 Burette reading 1st titration 2nd  titration 3rd  titration Final burette reading 25.60 24.60 24.00 Initial burette reading 2.50 1.80 1.05 Actual volume of acid 23.10 23.70 23.50

The equation for the reaction is

K2CO3(aq)   + H2SO4(aq)    →     K2SO4(aq)  + CO2(g )  + H2O(l)
[K   =  39,  O  =   16,  C  =  12,  H   =  1]
Average titre   =  23.10 + 23.70  +  23.50
3
=     70.3       =   23.4
3

(a)        List three errors in this table.                                                         [3 marks]
(b)        Mention
(i)    a suitable indicator for the experiment;
(ii)   the colour before and at the end of the titration.                    [3 marks]
(c)         Explain why the end point would not be affected if 5.00cm3 of distilled water were added to the titration mixture while the titration was in progress.
[1 mark]

(d)(i)    Calculate the number of moles of acid in the average titre recorded by the student.
(ii)    If 18.8g of impure  K2CO3  was used, determine the expected percentage
purity of  K2CO3  based on the student’s results.
[11 marks]

_____________________________________________________________________________________________________
OBSERVATION

This question was attempted by most candidates and the performance was good.
In part(a), candidates were able to list three errors in the table as follows:

1. absence of unit/no unit
2. values used for averaging were not concordant
3. arithmetic errors in the second/third titration.

In(b)(i), candidates correctly mentioned methyl orange/red or phenolphthalein as suitable indicator for the experiment.

In (b)(ii), some of the candidates could not give the colour of the indicator before the titration experiment.  However, a good number of them were able to give the colour at the end point.  The expected response from candidates was any of 1 or 2 as stated below.

SuSuitable indicator          Colour before titration     Colour at the end of titration

1.         Methyl orange/red      yellow                                       orange
2.         Phenolphthalein         red or pink                                colourless

In part (c), most candidates did not know that the end point will not be affected by addition  of 5.00cm3 of distilled water because the specific amount of the alkali in  25.0cm3  of  K2CO3((aq) remains the same.

In (d)(i) and (ii), candidates correctly calculated the number of moles of acid in the average titre recorded and determined the percentage purity of K2CO3  if  18.8g of   impure  K2CO3 was used by the student as follows.

1. 1000 cm3 contain 0.10 moles

∴   23.4 cm3  will contain  0.10 x 23.4    =    0.00234 mol
1000
(ii)              CA VA         =  1                 mole ratio
CB VB

CB      =     CA VA                                     making CB the subject of the formula
VB

=       0.10 x 23.4                         correct substitution
25
0.0936 moldm-3                          correct evaluation to 3 sig. figures and unit

OR
mole ratio of  K2CO3    :   H2 SO4    =     1 : 1

25cm3  of  K2CO3  contains  0.0234 moles

∴    1000cm3 of  K2CO3   will contain    1000 x 0.0234
25
=    0.0936 moldm-3

Molar mass of   K2CO3     =    78 + 12 +  48/138gmol-1
Mass of pure   K2CO3       =    0.09326 x 138
=    12.9g

%  Purity                            =   12 . 9  x  100
18.8
=   68.6%