Chemistry Paper 3 Questions and Answers - Achievers Joint Mock Exams 2023

INSTRUCTIONS

• Answer all the questions
• You are not allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper. This time is to enable you to read the question paper and make sure you have all the chemicals and apparatus you need.
• • All working must be clearly shown where necessary.

QUESTIONS

1. You are provided with:

• Solution A, Dilute hydrochloric acid
• Solution B, made by dissolving 0.5g of sodium hydroxide in water and made to 250cm³ of solution
• Solid C, Magnesium ribbon Phenolphthalein indicator

You are required to:

1. Standardize solution A
2. Determine the rate of reaction between solution A and magnesium

PROCEDURE- I

1. Measure exactly 10cm³ of solution A using a burette and transfer into 250ml
   volumetric flask.
2. Top up to the mark using distilled water. Label this solution D.
3. Drain the remaining solution A in the burette, rinse the burette thoroughly
   and fill the burette with solution D.
4. Pipette 25cm³ of solution B into a conical flask. Add three drops of phenolphthalein
   indicator
5. Titrate solution D with solution B and record your results in the table below
6. Repeat procedure (i) to (IV) to complete the table. (3 marks)

1. Calculate the average volume of solution D used (1 mark)
2. Calculate:
   1. Number of moles of solution B used (1½ marks)
   2. Number of moles of solution D in 250cm3 of solution (1½ marks)
   3. Morality of solutionA (1 mark)

PROCEDURE II

1. Cut solid C into equal pieces, each 2cm long
2. Using a burette, measure 12cm³ of solution A, into a clean boiling tube.
3. Drop one piece of solid C into the boiling tube containing solution A and start a stopwatch immediately. Stop the stopwatch when all solid C has just reacted. Record your results in the table below.
4. Repeat steps (ii) and (iii) above using 10cm³, 8cm³, 6cm³ and 4cm³ of solution A
5. Top up each with distilled water to make 12cm3 of solution and complete the
   table below (4mks

1. Plot a graph of I/t (y-axis) against the concentration of solution A (3 marks)
2. From the graph, determine the time taken for the reaction to reach completion when 1.5 moles of solution A are used (2 mk)
3. Comment on the shape of the graph (1 mk)
   

2. You are provided with solid Q. Carry out the tests below and record your  Observations and inferences in the spaces provided.

1. Strongly heat a spatula-end full of solid Q in a dry test tube
   

   Observation	Inference

2. Place the remaining solid Q in a boiling tube. Add 10cm3 of distilled water. Divide the solution into five portions.
   

   Observation	Inference

   1. To the first portion, add aqueous lead (II) nitrate solution
      

      Observation	Inference

   2. To the second portion add dilute nitric (V) acid, followed by barium nitrate solution
      

      Observation	Inference

   3. To the third portion add a few drops of sodium hydroxide until excess observation
      

      Observation	Inference

   4. To the fourth portion, add a few drops of aqueous ammonia until in excess
      

      Observation	Inference

   5. To the fifth portion, add a few drops of hydrochloric acid
      

      Observation	Inference

3. You are provided with solid R. carry out the tests below and record your observation
and Inference

1. Place a spatula-end full of solid R in a dry boiling tube and add about 10cm3 of distilled water. Shake thoroughly and heat to boil. Divide the solution into five portions.
   

   Observation	Inference

2.            
   1. Test the first portion with the universal indicator solution provided.
      

      Observation	Inference

   2. To the second portion, add a few drops of acidified potassium manganate (VII) solution
      

      Observation	Inference

   3. To the third portion, add a few drops of bromine water
      

      Observation	Inference

   4. To the fourth portion, add half spatula of sodium hydrogen carbonate
      

      Observation	Inference

   5. To the fifth portion in a boiling tube, add 5cm³ of ethanol followed by a few drops of concentrated sulphuric (VI) acid. Warm the mixture.
      

      Observation	Inference

MARKING SCHEME

1. You are provided with:

• Solution A, Dilute hydrochloric acid
• Solution B, made by dissolving 0.5g of sodium hydroxide in water and made to 250cm³ of solution
• Solid C, Magnesium ribbon Phenolphthalein indicator

You are required to:

1. Standardize solution A
2. Determine the rate of reaction between solution A and magnesium

PROCEDURE- I

1. Measure exactly 10cm³ of solution A using a burette and transfer into 250ml
   volumetric flask.
2. Top up to the mark using distilled water. Label this solution D.
3. Drain the remaining solution A in the burette, rinse the burette thoroughly
   and fill the burette with solution D.
4. Pipette 25cm³ of solution B into a conical flask. Add three drops of phenolphthalein
   indicator
5. Titrate solution D with solution B and record your results in the table below
6. Repeat procedure (i) to (IV) to complete the table. (3 marks)

Marks distributed as follows:

1. Complete table (1mk)
   • 3 tiration done (1 mk)
   • Incomplete table with 2 titration (½ mk)
   • Incomplete table with one titration done (0 mk)
     
     Penalties
   • Wrong arithmetic Inverted table
   • Unrealistic title values (unless explained)
   • Penalize ( ½ mk) for each to a miximum of (½ mk)
2. Decimal place (1mk)
   • Accept only 1 or 2 d.p used consistently, otherwise penalize fully.
   • Accept inconsistency in the use of zeros as initial burette reading e.g. 0.0, 0.00 or 0.000. NB decimal place tied to 1st and 2nd rows only.
3. Accuracy (1 mk)
   • Compare candidate’s title value with school value S.V. if one value within ± 0.1 of S.V (1mk)
   • No value within ± 0.1 of S.V butat least 1 value within ± 0.2 or S.V (0 mk)
4. Averaging (1 mk)
   • Values averaged must be sown
   • If 3 consistent titrations done and averaged = (1 mk)
   • If 3 titration done, but only 2 are consistent and averaged = (1 mk)
   • If only 2 titrations done, are consistent and averaged = (1 mk)
   • Otherwise penalize fully

1. Calculate the average volume of solution D used (1 mark)
   • 12.5+12.5+12.5 (1/2 mk)=12.5cm³
2. Calculate:
   1. Number of moles of solution B used (1½ marks)

      • Moles in 250cm³ = ^0.5/₄₀=0.0125 moles (1/2 mk)
        Moles used =^{(0.0125 ×25)}/₂₅₀ (1/2 Mk)=0.00125 moles

   2. Number of moles of solution D in 250cm3 of solution (1½ marks)
      • Moles of acid reacting = 0.00125(1/2 mk) mole ratio 1:1 250 → ?
        ^{(0.0125 ×25)}/₂₅₀ (1/2 mk)=moles (1/2 mk)
   3. Morality of solutionA (1 mark)

      • ^{(1000 ×0.25)}/₁₀ (1/2 mk)=25 moles(1/2 Mk)
        Molarity of solution A 10cm³ = 25 moles 1000 =?

PROCEDURE II

1. Cut solid C into equal pieces, each 2cm long
2. Using a burette, measure 12cm³ of solution A, into a clean boiling tube.
3. Drop one piece of solid C into the boiling tube containing solution A and start a stopwatch immediately. Stop the stopwatch when all solid C has just reacted. Record your results in the table below.
4. Repeat steps (ii) and (iii) above using 10cm³, 8cm³, 6cm³ and 4cm³ of solution A
5. Top up each with distilled water to make 12cm3 of solution and complete the
   table below (4mks

1. Plot a graph of I/t (y-axis) against the concentration of solution A (3 marks)
2. From the graph, determine the time taken for the reaction to reach completion when 1.5 moles of solution A are used (2 mk)
3. Comment on the shape of the graph (1 mk)
   
   

2. You are provided with solid Q. Carry out the tests below and record your  Observations and inferences in the spaces provided.

1. Strongly heat a spatula-end full of solid Q in a dry test tube
   

   Observation	Inference
   Colourless liquid condenses at coller parts of test tube Gas evolved turns red litmus blue A white residue remain	Hydrated salt NH+4 ions

2. Place the remaining solid Q in a boiling tube. Add 10cm3 of distilled water. Divide the solution into five portions.
   

   Observation	Inference
   Solid dissolves (½ mk) to form a colourless solution	Mg2+Al3+,Zn2+ Present Any One Or Cu2+, Fe2+, Fe3+ absent

   1. To the first portion, add aqueous lead (II) nitrate solution
      

      Observation	Inference
      A white precipitate form (½ mk)	CO2-3,SO4-2 .Cl-,SO32- present Any two ions award ( ½ mk) Penalize the ( ½ mk) for any contradictory ion.

   2. To the second portion add dilute nitric (V) acid, followed by barium nitrate solution
      

      Observation	Inference
      white precipitate is formed ( ½ mk)	SO2-4 ions confirmed ( ½ mk)

   3. To the third portion add a few drops of sodium hydroxide until excess observation
      

      Observation	Inference
      White precipitate ( ½ mk) dissolves in excess to form a colourless solution ( ½ mk)	Zn2+, Pb2+ or Al3+ Any 3 ions – (1mk) 2 ions ( ½ mk) 0 mk for any only one ion

   4. To the fourth portion, add a few drops of aqueous ammonia until in excess
      

      Observation	Inference
      White precipitate ( ½ mk) in excess	Pb2+, AI3+ present Both ions (1mk) One ion ( ½ mk)

   5. To the fifth portion, add a few drops of hydrochloric acid
      

      Observation	Inference
      No white precipitate formed (1 mk)	Pb2+ absent or Al3+ present (½ mk)

3. You are provided with solid R. carry out the tests below and record your observation
and Inference

1. Place a spatula-end full of solid R in a dry boiling tube and add about 10cm3 of distilled water. Shake thoroughly and heat to boil. Divide the solution into five portions.
   

   Observation	Inference
   Dissolves form (½mk) a colourless homogenous solution (½ mk)	Polar substance ( ½ mk)

2.            
   1. Test the first portion with the universal indicator solution provided.
      

      Observation	Inference
      PH value 1 – 3 ( ½ mk)	Strong acid present ( ½ mk)

   2. To the second portion, add a few drops of acidified potassium manganate (VII) solution
      

      Observation	Inference
      Purple acidified KMnO4 decolorised (1 mk)

   3. To the third portion, add a few drops of bromine water
      

      Observation	Inference
      Bromine water decolourised (1 mk)	OR –Oh. Present All three (1mk) Two only (½ mk)

   4. To the fourth portion, add half spatula of sodium hydrogen carbonate
      

      Observation	Inference
      Effervescence ( ½ mk)

   5. To the fifth portion in a boiling tube, add 5cm³ of ethanol followed by a few drops of concentrated sulphuric (VI) acid. Warm the mixture.
      

      Observation	Inference
      Sweet smelling compound formed (1 mk)