CHEMISTRY PAPER 3 - KCSE 2019 STAREHE PRE MOCK EXAMINATION (WITH MARKING SCHEME)

QUESTION 1

• You are provided with solution A and solution B
• Solution A is 1.25M hydrochloric acid
• Solution B is 0.16M hydroxide of metal M with formulae M(OH)_x where x is a whole number
• You are required to carry out the experiment to determine the value of x

PROCEDURE

1. Using a measuring cylinder, measure 50cm³ of solution A into a clean 250cm3 volumetric flask and make upto the mark with distilled water-label this solution W
2. Fill a clean burette with solution W
3. Pippette 25cm³ of solution B into a clean conical flask and add 2 drops of phenolphthalein indicator
4. Titrate solution B in the conical flask against solution A from the burette and record the results in the table below
5. Repeat [3] and [4] above as you record the results in the table below
   

   	I	II	III
   FINAL BURETTE READING [cm3]
   INITIAL BURETTE READING[cm3]
   VOLUME OF SOLUTION W USED

   
   
   1. Calcualte the;
      1. Average volume of solution W     [1mk]
      2. Concentration of solution W     [2mks]
      3. Number of moles of hydrochloric acid [in solution W] that reacted with each 25cm³ portion of the solution of the metal hydroxide [solution B]  [2mks]
      4. Number of moles of the metal hydroxide [solution B] that reacted with each portion of hydrochloric acid [solution W]         [2mks]
   2. Determine the stoichiometric equation of the reaction between the hydrochloric acid and the metal hydroxide solution     [2mks]
   3. Hence State the value of X              [2mks]

QUESTION 2

• You are provided with 0.5g of oxalic acid as solid C
• You are required to carry out the experiment to determine the solubility of the solid at different temperature

PROCEDURE

1. Fill a clean burette with distilled water
2. Put all the solid C into a clean boiling tube and then add 4cm³ of the distilled water from the burette
3. Heat the mixture as you stir gently with the thermometer until all the solid completely dissolves
4. Allow the solution to cool slowly as you continue stirring with the thermometer and note the temperature at which the crystals begins to form/just appears
5. Record this temperature in the table below
6. Add 2cm³ of distilled water from the burette to the same mixture in the boiling tube and repeat [3], [4] and [5] above
7. Add 2cm more of distilled water to the mixture and repeat 3,4 and 5 above again
8. Continue adding 2cm³ of distilled water as you repeat 3,4 and 5 above until the total volume of distilled water added to solid C is 12cm³
   

   Total volume of distilled water Added to solid C in cm3	Temp. in 0C at which crystals Of solid C appears/begin to form	Solubility of solid C in g Per 100g of water
   4
   6
   8
   10
   12

   1. Complete the table by filling the solubilities of solid C in g per 100g of distilled water [1mk]
   2. Using your table, plot the solubility curve of solid C by plotting the solubility of solid C against the temperature at which crystals firs appear
   3. From your graph, determine the solubility of solid C at;
      1. 68⁰C       [1mk]
      2. 62⁰C       [5mks]
   4. From your graph, explain the relationship between temperature and the solubility of solid C[1mk]

QUESTION 3

• You are provided with solid D and E
• You are required to carry out the tests below as you write down the observations and the inferences

1. SOLID D
   

   	TEST	OBSERVATION	INFERENCES
   (a) [i]	Scoop half of solid D with a metallic spatula and put into a Clean test tube Add about 2cm3 of distilled water and shake well. Divided the resulting mixture into 2 portions	[1mk]	[1mk]
   [ii]	To the 1st   portion, add NaOH[aq] drop wise till excess	[1mk]	[1mk]
   [iii]	Fetch a few drops of the 2nd portion on a metallic spatula and heat on a non-luminous flame	[1mk]	[ 1/2 mark]
   (b) [i]	Put the other half of solid D into another clean test tube and add HNO3 drop wise until there is no further change. Divide the resulting mixture into 2 portions	[1/2 mark]	[1/2 mark]
   [ii]	To the 1st portion, add 3 drops of Pb[NO3]2 [aq]	[1/2 mark]	[1mark]
   [iii]	To the 2nd portion, add three drops of Ba(NO3)2 (aq)	[1/2 mark]	[1/2 mark]

2. SOLID E
   

   	TEST	OBSERVATIONS	INFERENCES
   (a)	Scoop half of solid E on a metallic spatula and heat on a non-luminous flame	[1mk]	[1mks]
   (b) [i]	Put the remaining solid E into a clean test tube and add about 2cm3 of distilled water. Shake the mixture and divide into 2 portions	[1mk]	[1mk]
   [ii]	Determine the PH of the first portion	[1mark]	[1mark]
   [iii]	To the second portion, add 2 drops of bromine water and warm	[1mark]	[1mark]

   
   

MARKING SCHEME 

1. Table
   Complete table √1
   d.p√1
   P.O.A √1
   Accuracy √1
   NB:/Theoritically expected average value= 16.0cm³
   1.  
      1. average volume- tied to school value
      2.  
          
      3.  
          
      4.  
         
   2.  
      
      Where K is either ans in a(iii) or ans in a(iv) ( which ever of them is smaller)
      Nb: expected mole ratio of Hcl: M(OH)X= 1:1
      Thus stoichiometric equation is
      HCl_(aq) + MOH_(aq)  →MCl_(aq) + H₂O(L)√1
   3. value of X =1√1 (from the stoichiometric equition)

2. Table
   Expected values table
   

   Value of H2O	Temp ᴼC	Solubilities
   4	72	125
   6	68	83.33
   8	61	62.5
   10	54	50
   12	46	41.67

   
   Complete table     √1
   Trend- temperature gradually decreasing as volume of water added increases   √1
   d.p√1
   accuracy- tied to school value   √1
   1. refer to the table above
      √1for complete table
   2. solubility curve
      
   3.  
      1. correctly read value from graph   √1
      2. correctly read value from graph   √1
   4. Solubility of solid C increases with increase in temperature   √1
3.  
   1. Solid D
      

      	OBSERVATIONS	INFERENCES
      A(i)	Solid dissolves √to form a colourless solution√	Fe2+, Fe3+, Cu2+ absent     √1
      (ii)	No white ppt √	Ca2+, Mg2+, Al3+, Pb2+, Zn2+ absent√1
      (iii)	Burns √with a golden yellow   √	Na+confimed√
      b(i)	Effervescence occurs √	HCO-3, SO2-3 ,   CO2-3 Present √1
      (ii)	White ppt formed √	SO42-, Cl-√1
      (iii)	White ppt formed √	SO42- Confirmed √

   2. Solid E