Chemistry Questions and Answers - Form 3 Mid Term 1 Exams 2022

Questions

1. A substance contains 25.6% copper, 12.8% sulphur, 25.6% oxygen and 36.0% water of crystalisation. Calculate its simplest formula. (Cu=64, S=32, O=16 and H1) (4mks)
2. An organic compound has an empirical formula CH3O and relative molecular mass 62. What is its molecular formula? (2mks)
3.          
   1. State Charles’s law. (1mk)
   2. Draw a sketch graph to illustrate Charles’s law. (2mks)
   3. At a temperature of 57^OC, nitrogen gas occupies a volume of 750cm³. At what temperature will the gas occupy 100cm³? Express the answer in degrees Celsius. (3mks)
4. A given mass of a gas occupies 20cm3 at 25OC and 670mmHg pressure. Find out the volume it will occupy at;
   1. 10^OC and 335mmHg. (3mks)
   2. O^OC and 760mmHg. (3mks)
5. Write ionic equation for the reactions between:
   1. Barium chloride solution and copper (II) Sulphate solution. (2mks)
   2. Zinc and copper (II) sulphate. (2mks)
6. Zinc metal and hydrochloric acid reacts according to the following equation.
   Zn_(s) + 2HCl_(aq) → ZnCl_2(aq) + H_2(g)
   1.96g of Zinc metal were reacted with 100cm3 of 0.2m hydrochloric acid.
   1. Determine the reagent that was in excess. (Zn=65.4) (2mks)
   2. Calculate the total volume of hydrogen gas that was liberated at S.T.P (S.T.P = 22.4l) (2mks)
7. Complete the table below which show properties of indicators used in titrations. (3mks)
   ​

   Indicator	Colour in Acid	Colour un Alkali	Colour in neutral
   Phenolphthalein		Pink	Colourless
   Methyl orange	Pink		Orange
   Screened methyl orange	Red	Green

8. Solution A was made up by dissolving 2.65g, of a metal carbonate, X₂CO₃ in water and diluting the solution to 250cm³. B is a 0.25m solution of hydrochloric acid. 20cm³ portions of A were titrated with solution B using methyl orange indicator with the following results.
   1.  
      1. Complete the following table. (1 ½ mks)
         

         Burette readings	1st	2nd	3rd
         Final reading (cm3)			48.1
         Initial reading (cm3)	0.0	16.1	32.1
         Volume of solution B used (cm3)	16.1	16.0

      2. The volume of the pipette used. (1mk)
      3. Calculate the average volume of solution B used. (1mk)
   2. The equation of the reaction is;
      2HCl_(aq) + X₂CO_3(aq) → 2XCl_(aq) + H₂O_(l) +CO_2(g)
      Calculate the concentration of the solution A in;
      1. Moles/litre (3mks)
      2. g/litres (2mks)
   3. Calculate the Relative Atomic Mass of X. (C=12, O=16, X=?) (3mks)
9. State the Gay Lussac’s law. (1mk)
10. If it takes 30 seconds for 100cm³ of Carbon (IV) Oxide to diffuse across a porous plate. How long will it take 150cm³ of nitrogen (IV) oxide to diffuse across the same plate under similar condition? (C=12, N=14, O=16) (3mks)
11. A form three student set up an experiment to explain the rate of diffusion of ammonia and hydrogen chloride in air. Study it and answer the question that follows.
    
    
    1. What observation is made in the glass tube? Explain? (2mks)
    2. Determine the molecular masses of ammonia (NH3) and hydrogen chloride (HCl) (N=14, H=1, Cl=35.5) (1mk)
    3. Which gas covered a longer distance? ( ½ mk)
12. Name two laboratory apparatus that are used to measure fairly accurate volumes of liquids. (2mks)

Marking Scheme

1. A substance contains 25.6% copper, 12.8% sulphur, 25.6% oxygen and 36.0% water of crystalisation. Calculate its simplest formula. (Cu=64, S=32, O=16 and H1) (4mks)
   

   Elements	Cu	S	O	H2O
   % by mass	25.6	12.8	25.6	36.0
   R.A.M/RFM	64	32	16	18
   Ratio of moles	25.6=0.4  64	128=0.4  32	25.6=1.6  16	36=2 18
   Simplest ratio of moles	0.4 =1  0.4	0.4 =1  0.4	1.6 =4   4	2  =5  0.4

   Empirical formula CuSO₄.5H₂O
   
   
2. An organic compound has an empirical formula CH3O and relative molecular mass 62. What is its molecular formula? (3mks)
   • Relative E.F mass = 12 + 3 16 =31
     (CH3O)n = 62
     n=   62   = 2
           31
     Molecular formula is C₂H₆O₂
     
     
3.        
   1. State Charles’s law. (1mk)
      • The volume of a given mass of a gas is directly proportional to its absolute temperature, its pressure being kept constant
        
        
   2. Draw a sketch graph to illustrate Charles’s law. (2mks)
      
   3. At a temperature of 57OC, nitrogen gas occupies a volume of 750cm3. At what temperature will the gas occupy 100cm3? Express the answer in degrees Celsius. (3mks)
      •  V1 =    V2           V1=   750cm³                   V2=100cm³
        T1         T2           T1= 57 + 273 = 330K        T2=?
        
         750 =  100            T2 = 44K = - 229^OC
        330        T2               t = T - 273
        T2= 100 x   330  = 44-273
                          750
        
        
4. A given mass of a gas occupies 20cm3 at 25^OC and 670mmHg pressure. Find out the volume it will occupy at;
   1. 10^OC and 335mmHg. (3mks)
      • P1V1 = P2V2
           T1       T2
        P1= 670
        V1= 20cm³
        T1= 25 +273 = 298K
        P2= 335mmHg
        V2=?
        T2=10+273=283K
        
        670x20 = 335 x V2
            298         283
        V2= 670 x 20 x    283  
                    298          335
        V2 = 38cm³
        
        
   2. O^OC and 760mmHg. (3mks)
      • P1V1 = P2V2
            T1        T2
        P1= 670mmHg
        V1= 20cm³
        T1= 25 + 273=298K
        P2= 760mmHg
        V2 = ?
        T2= 0+273 =273K
        
        670 x 20 = 760 x V2
              298          273
        V2= 670 x 20 x   273
                    298         760
        V2= 16cm³
        
        
5. Write ionic equation for the reactions between:
   1. Barium chloride solution and copper (II) Sulphate solution. (2mks)
      • BaCl_2(aq) + CuSO_4(aq) → BaSO_4(s)+ CuCl_2(aq)
        Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s)
        
        
   2. Zinc and copper (II) sulphate. (2mks)
      • Zn_(s) + CuSO_4(aq) → ZnSO_4(aq)+ Cu_(s)
        Zn_(s) + Cu²⁺_(aq) → Zn²⁺_{(aq )} + Cu_(s)
        
        
6. Zinc metal and hydrochloric acid reacts according to the following equation.
   Zn_(s) + 2HCl_(aq) → ZnCl_2(aq) + H_2(g)
   1.96g of Zinc metal were reacted with 100cm³ of 0.2m hydrochloric acid.
   1. Determine the reagent that was in excess. (Zn=65.4) (2mks)
      • Moles of Zn =   1.96   = 0.03
                                 65.4
        Moles of HCl = 0.02 → 1000cm³
                                   ?    →  100cm³
        0.02 x 100 = 0.002 of HCl
           1000
        Zinc was in excess since 0.002 moles of HCl needs to react with 0.001 moles of Zinc.
        
        
   2. Calculate the total volume of hydrogen gas that was liberated at S.T.P (S.T.P = 22.4l) (2mks)
      • Moles of H2= 0.001
        Volume = 1 mole → 22.4
                       0.001 mole → ?
        = 0.001 x 22.4
                       1
        = 0.0224 litre
        
        
7. Complete the table below which show properties of indicators used in titrations. (3mks)
   

   Indicator	Colour in Acid	Colour un Alkali	Colour in neutral
   Phenolphthalein	Colourless	Pink	Colourless
   Methyl orange	Pink	Yellow	Orange
   Screened methyl orange	Red	Green	Grey

8. Solution A was made up by dissolving 2.65g, of a metal carbonate, X2CO3 in water and diluting the solution to 250cm3. B is a 0.25m solution of hydrochloric acid. 20cm3 portions of A were titrated with solution B using methyl orange indicator with the following results.     
   1.  
      1. Complete the following table.
         

         Burette readings	1st	2nd	3rd
         Final reading (cm3)	16.1	32.1	48.1
         Initial reading (cm3)	0.0	16.1	32.1
         Volume of solution B used (cm3)	16.1	16.0	16.

      2. The volume of the pipette used. (1mk)
         • 20cm³
           
           
      3. Calculate the average volume of solution B used. (1mk)
         • = 16+16+16.1
                      3
           = 16.0cm³
           
           
   2. The equation of the reaction is;
      2HCl(aq) + X2CO3(aq) → 2XCl(aq) + H2O(l) +CO2(g)
      Calculate the concentration of the solution A in;
      1. Moles/litre (3mks)
         • 2HCl + X₂CO₃
           Ratio 2:1
           Moles of HCl used = 16 x 0.25 = 0.004
                                                 1000
           Moles of X2CO3 used = ½ x 0.004 = 0.002
           20cm3 → 0.002 moles
           1000cm3 → ?
           0.002 x 1000 = 0.1M
                  20
      2. g/litres (2mks)
         • 250cm³ → 2.65g
           100cm³ → ?
           2.65 x 1000 = 10.6g/L
               250
           
           
   3. Calculate the Relative Atomic Mass of X. (C=12, O=16, X=?) (3mks)
      • molar mass of A =    g/litre   
                                       morality
        =  10.6  
           0.1
        = 106
        Molar mass of X₂CO₃
        = (2xx) + 12 + (3x16) =106
        2x=106-60
        2x=46
        X=23
        
        
9. State the Gay Lussac’s law. (1mk)
   • When gases react; they do so in volumes that bear a simple ratio to one another and to the volumes of the produce if gaseous, temperature and pressure remaining constant.
     
     
10. If it takes 30 seconds for 100cm3 of Carbon (IV) Oxide to diffuse across a porous plate. How long will it take 150cm3 of nitrogen (IV) oxide to diffuse across the same plate under similar condition? (C=12, N=14, O=16) (3mks)
    • 100cm3 → 30 seconds of CO2
      150cm3 → ?
      30 x 150
         100
      = 45 seconds
      TCO₂
      TNO_{2
      √}(M_rCO₂/M_rrO₂)
         45   = _√⁴⁴/₂₈
      TNO₂ 
        45   = 0.978
      T_rO₂
      T_rO₂=    45   
                  0.978
      T_rO₂ = 46 seconds
11. A form three student set up an experiment to explain the rate of diffuse of ammonia and hydrogen chloride in air. Study it and answer the question that follows.
    1. What observation is made in the glass tube? Explain? (2mks)
       • White solid is formed, because concentrated ammonia solution generates ammonia and concentrated hydrochloric acid generated hydrogen chloride gas which reacted to form solid ammonia chloride.
         
         
    2. Determine the molecular masses of ammonia (NH3) and hydrogen chloride (HCl) (N=14, H=1, Cl=35.5) (1mk)
       • NH₃= 12 + (1 x 3) = 15
         HCl = 1 + 35.5 = 36.5
         
         
    3. Which gas covered a longer distance? ( ½ mk)
       • NH₃ gas
         
         
12. Name two laboratory apparatus that are used to measure fairly accurate volumes of liquids. (2mks)
    • Volumetric flasks
    • Syringes
    • Pipettes
    • Burettes