CHEMISTRY - FORM 3 END TERM 1 EXAMS 2020

1. An oxide of element D has the formulae D₂O₃
   1. What is the oxidation number of D? (1mk)
   2. Write the formulae of the hydroxide of D? (1mk)
2. The diagram below shows the bonding present in water molecules
   
   Name the type of bond indicated by
   1. X (1mk)
   2. Y (1mk)
3. When crystals of hydrated iron (II) sulphate, FeSO₄.7H₂O were left in the open air overnight, they changed to light green powder
   1. Explain the observations above (1mk)
   2. Name the process involved (1mk)
4. Distinguish between hydrated salt and anhydrous salt (2mk)
5. Study the set up below and answer the questions that follow
   
   1. State the observations made in
      1. Tube A (1mk)
      2. Tube B (1mk)
   2. Write a chemical equation for the reaction that took place in tube A. (1mk)
6.  
   1. State Boyles law (1mk)
   2. A sample of a gas occupies 800cm³ at 50^oc and 760mmHg pressure. Determine the volume the gas would occupy at 0^oC and 720mmHg pressure (3mk)
7. Burning magnesium ribbon continues to burn in a gas jar of Carbon (IV) Oxide. Explain (3mk)
8.  
   1. State Grahams law of diffusion (1mk)
   2. The set up below was used to study the rate of diffusion of hydrogen and carbon (IV) Oxide
      
      State and explain the observation which were made on the coloured liquid (3mk)
9. A compound has the following percentage composition by mass: carbon 40%, hydrogen 6.7% and oxygen 53.3%. The relative formula mass, RFM of the compound is 180. Determine its;
   1. Empirical formula   (C=12, H=1, O=16) (2mks)
   2. Molecular formula (2mk)
10. Describe how you would obtain a pure sample of lead (II) carbonate from a solid mixture of lead (II) carbonate and sodium carbonate. (3mks)
11.  
    1. Define the term electrolyte (1mk)
    2. The set up below was used to electrolyse lead(II) bromide using graphite electrodes
       
       1. Label the cathode on the diagram above (1mk)
       2. Write the ionic equation for the reaction that occurs at the
          1. Anode (1mk)
          2. Cathode (1mk)
       3. What observations would be made on the bulb if the source of heat was withdrawn? Explain. (2mks)
12. A solution contains 4g of Sodium hydroxide in 250 cm³ of solution. Determine the molarity of the solution. (Na=23, O=16, H=1) (3mks)
13. The grid below represents part of the periodic table. The letters are not the actual symbols of the elements.
    
    1. Select the most reactive non metal (1mk)
    2. Select an element which
       1. Can form an ion of charge +2 (1mk)
       2. Has the least ionisation energy (1mk)
    3. Write the formula of the compound formed when E and F combine (1mk)
    4. What type of bond is formed when J and C combine? Explain (2mk)
    5. Compare the atomic radius of B and J. Give a reason for your answer (2mk)
    6. An atom of element W is denoted by ¹⁴₇ Indicate its position in the grid by writing letter W (1mk)
14. A certain volume of carbon (IV) oxide takes 200 seconds to diffuse through a porous plug. How long would it take the same volume of Ozone, O₃ to diffuse under the same conditions? ( C=12, O=16) (3mk)
15. Two aqueous solutions of lead(II) nitrate and sodium chloride were mixed together
    1. Write the stoichiometric equation for the reaction that took place (1mk)
    2. Write the ionic equation for the reaction (1mk)
16. The diagram below shows a charcoal ‘jiko’ with different regions
    
    1. Write an equation for the formation of the product in region B. (1mk)
    2. What observations are made in region C? Explain (2mk)
17. Determine the number of molecules of oxygen gas contained in 11.2g of oxygen gas. ( O=16, Avogadro’s constant= 6.02 X 10²³) (3mk)
18. The table below shows the PH values of solutions A, B, C and D
    

    SOLUTION	A	B	C	D
    PH	6.5	13.0	8.0	2.0

    1. Which of the above solutions is likely to be dilute hydrochloric acid? (1mk)
    2. Name the type of reaction that takes place when B reacts with D (1mk)
    3. Which solution would be advisable for use by a patient with indigestion? (1mk)
19. Complete the chemical equations below to show the decomposition of certain salts.
                             _heat
    
    1. NaHCO_3(s)   →      (1mk)
                         _heat
    2. Zn(NO₃)_2(s)  →    (1mk)
20. Zinc metal was reacted with 200cm³ of 0.1 M hydrochloric acid according to the following equations
    Zn (s) + 2HCl _(aq)     →     ZnCl_2(aq) +H_{2 (g)}
    Calculate the:
    1. Number of moles of hydrochloric acid used (2mk)
    2. Volume of hydrogen gas produced at s.t.p. (molar gas volume at s.t.p=22.4 litres) (2mk)
21. The set up below was used to prepare oxygen in the laboratory
    
    1. Complete the diagram above to show how oxygen is collected (2mk)
    2. Identify the liquid Q (1mk)
    3. State one commercial use of oxygen (1mk)
22. Study the table below and answer the questions that follow
    

    Substance	Melting point (0c)	Boiling Point (0c)	Electrical conductivity
    			In solid	In liquid
    U	1083	2595	good	good
    V	801	1413	Poor	good
    W	5	80	poor	poor
    X	-115	-84	poor	poor
    Y	3550	4827	poor	poor

    1. Which substance is likely to have the following structures
    2. Which substance is likely to be a metal? (1mk)
       1. Simple molecular (1mk)
       2. Giant ionic   (1mk)
23. 25cm³ of 0.1M potassium hydroxide solution was pipetted into a conical flask and titrated with dilute sulphuric (VI) acid. The results were tabulated as shown below.
    

    	I	II	III
    Final burette reading (cm3)	24.1	48.1	24.4
    Initial burette reading (cm3)	0.0	24.1	0.0
    Volume of acid used (cm3)	24.1	24.0	24.4

    1. Calculate the average volume of the acid used (1mk)
    2. Calculate the number of moles of potassium hydroxide used (2mk)
    3. Determine the concentration of sulphuric (VI) acid used in moles per litre. (3mk)

FORM 3 CHEMISTRY

MARKING SCHEME

1.  
   1. +3 1 mk
   2. D(OH) ₃          1 mk
2.  
   1. Hydrogen bonding 1mk
   2. Covalent bond             1 mk
3.  
   1. The crystals lost the water of crystallisation
   2. Efflorescence
4. Hydrated salt-contains water of crystallisation. 1 mk
   Anhydrous salt- does not contain water of crystallisation.           1 mk
5.  
   1.  
      1. green solid changes to black 1 mk
      2. White precipitate formed              1 mk
   2. CuCO_3(s) → CuO_(s) + CO_{2 (g)}
6.  
   1. The volume of a fixed mass of gas is inversely proportional to its pressure at constant temperature.
   2. P₁V₁ =  P₂V₂                    ½ mk
      T₁            T₂
      T₁ = 50+273= 323K)    
      T₂   = 0+273=273K)                         ½mk
      V₂ =760x800x273
              323x720                                  1 mk
      = 713.73cm³                                           1 mk

7. The heat (1mk) of burning magnesium decomposes CO₂ into carbon and oxygen 1mk
   The oxygen formed supports the burning of magnesium.
8.  
   1. The rate of diffusion of a gas is inversely proportional to the square root of its density at constant temperature and pressure
   2. Level of coloured liquid rose in the delivery tube.
      This is because H₂ being less dense/lighter than CO₂ diffuses faster out of the porous pot.
9.  
   1.  C          H         O
      Mole        ⁴⁰/₁₂    ^6.7/₁     ^53.3/₁₆    ½mk
                  = 3.333  = 6.7   = 3.331 ½ mk
      Ratio          1          2           1     ½ mk
      E.F   =     CH₂O                          ½ mk
   2. (CH₂O) _n= 180
      30n     = 180
      n         = 6                               1 mk
      M.F   =   C₆H₁₂O₆                     1 mk
10. Add water to the mixture and stir ½ mk
    Na₂CO₃ dissolves while PbCO₃ does not             ½ mk
    Filter to obtain PbCO3 as residue                         1 mk
    Wash residue with distilled water and dry between filter papers.                        1 mk
11.  
    1. Is a substance that allows electric current to pass and is decomposed by it.
    2.  
       1. Left –hand-side electrode
       2.  
          1. 2Br^-_(l) → Br_2(g) + 2e^-
          2. Pb²⁺_(l) + 2e^- → Pb_(s)
       3. Bulb goes off. (1mk) Lead (II) bromide becomes solid (1/2 mk) hence no free and mobile ions  ½ mk
12. RMM NaOH=23+16+1=40 ½ mk
    1 mole of NaOH     =    40g
    ?                          =    4g
    1x4
    40     ½ mk    = 0.1 moles      ½ mk
    0.1 moles = 250cm³
         ?        =  1000cm³
    0.1x1000                             1 mk                        
     250
    = 0.4M ½ mk

13.  
    1. C 1mk
    2.  
       1. B or J          1 mk
       2. G   1 mk
    3. EF₃ (reject F₃E)
    4. Ionic / electrovalent bond                   1 mk
       Formed when J loses electrons and C gains the electrons.   1 mk
       (reject: becauseJ is a metal and C a non metal )
    5. J has larger atomic radius than B (1mk)( or B has smaller atomic radius than J)
       J has more energy levels than B                     1mk
    6. Group V and period 2 in the periodic table.
14.   
    

15.  
    1. Pb(NO₃)_{2 (aq)} + 2NaCl _(aq)   →   PbCl_2(s) + 2NaNO_3(aq)
       
    2. Pb²⁺_(aq) + 2Cl^- → PbCl_2(s)
16.  
    1. CO_{2(g )}+ C_(s)  → 2CO_(g)
       
    2. blue flame                    1 mk
       The carbon (II) Oxide / CO produced burns in air.                      1 mk
17. RMM   O₂=32
    1 mole of O₂   =  32g
    ?                   =  11.2g
    1x11.2
    32                             1 mk
    = 0.35 moles.              ½ mk
    1 mole   = 6.02x10²³ molecules
    0.35 moles= 0.35x6.02 x 10²³ 1 mk
    = 2.107x10²³ molecules.                          ½ mk
18.  
    1. D 1 mk
    2. Neutralisation reaction                        1mk
    3. C                            1 mk

19.  
    1. 2NaHCO_3(s)    → Na₂CO_3(s) + CO_{2(g )}+ H₂O
    2. 2Zn(NO₃)₂(s) → 2ZnO_(s) + 4NO_{2(g) +} O_2(g)
20.  
    1. 0.1 moles= 1000cm³
                       = 200cm³
       0.1x200
       1000
       =0.02 moles
    2.  
       

       HCl : H2	1 mole -----------22.4 litre
       Mole ratio 2 : 1	0.01 moles -------   0.01 x 22.4     1 mk
       0.02 : 0.01 moles	=   0.224 litres     ½ mk
       	(Or 224 cm3)

21.  
    1.  
       
       NB: method                                       1 mk
       Working                                    1 mk
    2. Water / H₂O
    3.  
       1. Welding and cutting metals.
       2. By patients with breathing problems.
       3. By deep sea divers.
       4. By mountain climbers.
22.  
    1. Substance U
    2.  
       1. X
       2. V
23.  
    1.  1+24.0 = 24.05 cm³
           2 
    2. 0.1 moles of KOH   =  1000cm³
       ?                         =    25cm³
       0.1x25
       1000        
       =   0.0025 moles
    3. 2KOH_(aq)   +   H₂SO_{4 (aq)}   →   K₂SO_4(aq)   +   2H₂O_(l)
       Mole ratio   2 :  1
       0.0025 : 0.0025   = 0.00125
                        2
       0.00125 moles = 24.05 cm³
       ?                    = 1000cm³
       0.00125x1000
       24.05
       = 0.052M        ½ mk