Pre-KCSE 2013

You are provided with :

  • Solution A containing an oxidizing agent A;
  • Solution B ,0.05M aqueous sodium thiosulphate;
  • Solution C containing a reducing agent C;
  • Aqueous Potassium iodide;
  • Solution D, starch solution.

You are required to determined the:

          Concentration of solution A

          Rate of reaction between the oxidizing agent A and the reducing agent C.

Procedure 1

  1. Using a pipette and pipette filter ,place 25.0cm3 of solution A into a 250ml conical flask.
  2. Meassure 10cm3 of aqueous potassium iodide and add it to solution A in the conical flask.

Shake the mixture .Add 10cm3 of 2M sulphuric(VI)acid to the mixture and shake.

  • Fill a burette with solution B and use it to titrate the mixture in the conical flask until it just turns orange yellow.Add 2cm3 of solution D to the mixture in the conical flask .Shake thoroughly. Continue titrating until the mixture just turns colourless. Record your results in table 1 below.
  • Repeat the procedure and complete table 1.Retain the remainder of solution A and D for use in procedure II

Table I

          I        II       III
Final  burette reading20.020.020.0  
Initial burette reading0.00.00.0  
Volume of solution B used (cm3)  


(a)Calculate the:

          (i) average volume of solution B used(1mk)

20.0 + 20.0 +20.0 √    =   20.0 √cm3


          (ii)number of moles of sodium thisulphate(1mk)

Moles = molarity x burette volume


            => 0.05 x20.0 √  =  0.001 / 1.0 x10-3 √ moles


(b)Given that one mole of A reacts with six moles of sodium thiosulphate ,calculate the:

(i)number of moles of A that were used (1mk)

Mole ratio A:B = 1:6 √

    => Moles A  =    0.001 / 1.0 x10-3 moles = 0.00016/1.6 x10-4 √ moles


(ii)concentration of solution A in moles per litre(2mk)

Molarity of solution A = moles x 1000

Pipette volume

=> 0.00016/1.6 x10-4 moles x1000  √    =  0.008/8.0 x10-4 M√


Procedure II

  1. Label six test tubes as 1,2,3,4 ,5 and 6 and place them on a test tube rack.
  2. Using a clean burette, measure the volumes of distilled water as shown in table 2 into  the labeled test tubes.
  3. Using a burette ,measure the volumes of solution A shown in table 2 into each of the test tubes .
  4. Clean the burette and rinse it with about 5cm3 of solution C
  5. Using the burette ,measure 5cm3 of solution C and place it into a 100ml beaker.
  6. Using a 10ml measuring cylinder ,measure 5cm3 of solution D and add it to the beaker containing solution C .Shake the mixture.
  7. Pour the contents of test tube number 1 to the mixture in the beaker and immediately start  off stop watch/clock. Swirl the contents  of the beaker.Record the time taken for a blue colour to appear in table 2.
  8. Repeat steps 5 to7 using the contents of test-tube 2,3,4,5 and 6.
  9. Complete table 2 by computing Rate =      1        (S-1)


Table 2(Sample results)

Test-tube number123456
Volume of distilled  water(cm3)023567
Volume of solution A(cm3)  1087543
Rate =                 1     (S-1                         time0.025 2.5 x 10-20.0167 1.67 x 10-20.0143 1.43 x 10-20.0111 1.11 x 10-20.0.1 1.11 x 10-20.0083 8.3 x 10-3

Plot a graph of rate(y-axis )against volume of solution A(3mk)

(b)What time would be taken for the blue colour to appear if the experiment was repeated using 4cm3 of distilled  water and 6cm3 of solution A?(2mk)

From a correctly plotted graph

1/t  at 6cm3 = 0.0125 √  =>              t = 1/0.0125 = 80seconds√

2. You are provided with solid E. Carry out the experiments below. Write your observations and inferences in the spaces provided

(a)Place all solid in a boiling tube .Add 20cm3 of distilled water and shakeuntil all the solid dissolves.Label the solution as solution E.Use solution E for experiment (i)and (ii)

(i)To 2cm3 of solution E in a test tube in each of experiment I,II,III and IV add:

I. two drops of aqueous sodium sulphate(VI)

Observation(1mk)                                                       Inferences(1mk)

White precipitate                                               Pb2+   Ba2+   Ca2+

II.five drops of aqueous sodium chloride

Observations (1mk)                                                     Inferences(1mk)

White  ppt                                                                    Ca2+  Ba2+

III.two drops of barium chloride

Observations(1mk)                                           Inferences(1mk)

No white ppt                                               SO42-   SO32-   CO32-                                                                                                                                                                                                                                                                                                                                                                                         IV.two drops of lead(II)nitrate(V)  

Observations(1mk)                                           Inferences(1mk)

No white ppt                                                             Cl                                                                                                                                                                                       

(ii)To 2cm3 of solution E in a test  tube  ,add 5 drops of aqueous sodium hydroxide .Add the piece of aluminium foil provided to the mixture and shake. Warm the mixture and test any gas produced with both blue and red litmus papers

Observations(1mk)                                           Inferences(1mk)

Blue litmus paper remain blue

Red limus paper turn blue                                NO3         

Effervescence /fizzing/ bubbles


Solid E is Calcium  nitrate(V) / Barium nitrate(V)

3.You are provided with solid F. Carry out the following  test.Write your observations and inferences in the spaces provided.

(a)Place all of solid F in a boiling tube. Add about 20cm3 of distilled water and shake until all the solid dissolves. Label the solution as solution F.

Add about half of the solid sodium hydrogen carbonate provided to 2cm3 of solution F

Observations(1mk)                                                     Inferences (1mk)

        No effervescence/fizzing                                     H+    absent

(b)(i)Add about 10cm3 of dilute hydrochloric acid to the rest of solution  F in the boiling tube. Filter the mixture. Wash the residue with about  2cm3 of distilled water.Dry the residue between  filter papers. Place about  one third of the dry residue on a metallic spatula and burn it in a Bunsen burner flame.

Observations(1mk)                                                        Inferences(1mk)

Solid burns with a yellow  sooty flame            C      C           //    C       C

(ii)Place all the remaining residue into a boiling tube. Add about 10cm3 of distilled water and shake thoroughly. Retain  the mixture for the tests in (c)

Observations (1/2mk)                                                      Inferences(1/2mk)                                                                                                                          Solid dissolves to a colourless solution          Polar compound                                                                                                                                                                           

(c)Divide the mixture into two portions:

(i)to the first portion ,add the rest of the solid sodium hydrogen carbonate

Observations (1mk)                                                        Inferences(1mk)                                                                                                                                    Effervescence/fizzing                              H+                                                                                                                                                          

(ii)to the second portion ,add two drops of bromine water.

Observations (1mk)                                                        Inferences(1mk)                                                                                                                            Bromine water decolorized                       C     C //      C      C      bonds