Chemistry Paper 3 Questions and Answers With Confidential - Wahundura Boys Mock Examination 2023

QUESTIONS

1. You are provided with
   • A monobasic acid, Solid T
   • Sodium hydroxide, solution B.
   • 01M solution R of a dibasic acid H₂A.

You are required to:

• Prepare a saturated solution of solid T.
• Standardize sodium hydroxide solution B.
• Determine the solubility of solid T in water at room temperature.

PROCEDURE

• Place all the solid T provided into a dry conical flask. Measure out 100cm³ of distilled water using a measuring cylinder and add it to solid T. Shake thoroughly and leave it to stand. RETAIN IT FOR USE IN PROCEDURE (iii)
• Fill the burette with solution B, pipette 25cm³ of solution R into a conical flask. Add 2 or 3 drops of phenolphthalein indicator and titrate against solution B until a permanent pink colour appears. Record the readings in the table below, repeat to obtain three accurate readings.

                           TABLE 1                                     4MKS

1. Calculate the average titre. (1 mark)
2.  
   1. Write the equation for the reaction of the dibasic acid and sodium hydroxide. (1 mark)
   2. Calculate the concentration of sodium hydroxide, solution B in moles per litre. (2 marks)
3. Measure the temperature of the solution of solid T. Using a dry filter paper and dry filter funnel, filter the solution into a dry conical flask. Pipette 25cm³ of the filtrate into a conical flask. Add 2 or 3 drops of phenolphthalein indicator and titrate against sodium hydroxide, solution B until a permanent pink colour appears. Record the readings in the table below. Repeat to obtain three accurate readings. Temperature of the solution of solid T = ______________ ̊ (1 mark)

TABLE 2

                                                                                                                     (4 marks

4. Calculate the average titre of solution B used. (1 mark)
5. calculate:
   1. The number of moles of acid T in 25cm³ of the filtrate. (2 marks)
   2. The number of moles of acid T in 100cm³ of solution T. (1 mark)
   3. Given that the molecular formula of acid T is C₇H₆O₂ , calculate the solubility of the acid in grammes per 100cm3 of water. ( C=12.0 , H=1.0 , O=16.0 ) (1 ½ marks)

You are provided with

• Magnesium ribbon labelled solid C.
• 0 M sulphuric (vi) acid labelled solution D.
• A stop watch.
  You are required to determine the rate of reaction between magnesium and sulphuric (vi) acid at different concentrations.

PROCEDURE

Place 5 test tubes in a test tube rack and label them 1,2,3,4 and 5.  Using a 10cm³ measuring cylinder, measure out the volume of 2.0 M sulphuric (vi) acid solution D as shown in the table below. Pour in the corresponding test tubes. To each test tube, add the volume of water as shown in the table below. Cut out five pieces of magnesium ribbon, solid C each exactly 1 cm long. Transfer all the solution in test tube 1 into a 100cm³ beaker. Place one piece of magnesium into the beaker and start a stop watch immediately. Swirl the beaker continuously ensuring that the magnesium is always inside the solution. Record the time taken for magnesium to disappear in table 3 below. Wash the beaker and repeat the procedure above for each of the solution in test tube 2, 3, 4 and 5 and complete table 3.

TABLE 3                                                                                    (3 ½ MKS)

1. 1.  Plot a graph of rate of reaction of () (y-axis) against volume of solution D. (3 marks)
   2. Use the graph to determine the time that would be taken for a 1 cm length magnesium ribbon to disappear if the volume of the acid solution D used was 8.5 cm³.
   3. In terms of rates of reactions. Explain the shape of your graph. (1 mark)

3. Place a spatula end full of solid G in a boiling tube and add 100cm³ of distilled water while shaking. Divide the solution into four portions.

1. to the first portion, add sodium hydroxide dropwise till in excess.

2. To the second portion, immerse a glass rod into the solution and place the wet rod into a non-luminous flame of a Bunsen burner.

3. To the third portion, add a few drops of barium nitrate solution followed by dilute nitric acid.

2. You are provided with solid H which is to be dissolved in about 6cm³ of distilled water.  Divide the resulting solution into 5 portions carry out the tests described and note down

      your observations and inferences.

1. To the 1^st portion add 2-3 drops of acidified potassium dichromate (vi).

2. To the second portion add 2-3 drops of acidified potassium manganate (vii)

3. To the third portion add 2-3 drops of bromine water.

4. Place a quarter spatula full of sodium hydrogen carbonate in a dry test tube. Add the 4^th portion to the NaHCO₃ in the test tube.

CONFIDENTIAL

In addition to the apparatus and fittings found in the chemistry laboratory,each candidate should be provided with the following:

• 1.5g solid T accurately weighed
• About 250cm³ of solution B
• About 150cm³ of solution R
• Distilled water
• One burette
• One pipette
• 100cm³ measuring cylinder
• One filter paper
• One filter funnel
• 2 conical flasks (250 ml)
• One Thermometer
• Phenolphthalein indicator
• Solid C(About 6CM long magnesium ribbon)
• Solution D about 50 cm³ of 2.0M Sulphuric (IV) acid
• Stop watch
• One 10cm³ measuring cylinder
• One 100cm³ glass beaker
• 6 clean test tubes
• One boiling tube
• About 0.5g of solid G
• About 0.5g of solid H
• Bunsen burner
• A glass rod
• About 0.5g of NaHCO₃

ACCESS TO

• 2M Sodium hydroxide
• 2M Ammonium hydroxide
• Aqueous Barium nitrate
• 2M Nitric acid
• Acidified Potassium chromate (VI)
• Acidified Potassium manganate (VII)
• Bromine water

NOTE
Solid T = Benzoic acid

Solution R(0.01M H2SO4)

1. Dissolve 56cm³ of conc H₂SO₄ in about 500cm3 of distilled water. Shake well and make up to 1 litre.
2. Take 10 cm³ of the solution prepared in (i) above and dilute it with distilled water to make it up to 1 litre of solution R.

Solution B(0.02M NaOH)

1. Dissolve 4.0g of NaOH in distilled water and make it up to 1 litre of solution.
2. Take 200 cm³ of the solution prepared in (i) above and dilute it with distilled water to make it up to 1 litre of solution B.
   Solid G= Sodium carbonate
   Solid H= Oxalic acid
   Solid C= Magnesium ribbon
   Solution D= 2.0M Sulphuric (VI) acid

Marking scheme

1.  
   1. complete table………………..1mk
      decimal point…………………..1mk
      accuracy ………………………....1mk
      principles of averaging……..1mk
      final accuracy……………………..1mk
   2.  
      1. H₂A(aq) + 2NaOH(aq)→ Na₂A(aq) +H₂O(l)
         Note Penalize ½ mk if state symbols missing otherwise penalize Fully for wrong or unbalanced equation
      2. concentration of NaOH
         Method 1.                  Method 2
         M1V1 = ½                 Moles of acid = 0.01×25
         M2V2                                                      1000
         = 0.01×25                 Moles of NaOH = 0.00025 × 2
         M2 × Av Titre                           = 0.0005 moles
         = 0.01 × Av Titre     Molarity 0f NaOH =0.005 × 1000
                   25                                               Av Titre
         = correct ans                                  =correct ans
         (b)(iii)and (iv) (Mark table 2 as table1 above)
         complete table………………..1mk
         decimal point…………………..1mk
         accuracy ………………………....1mk
         principles of averaging……..1mk
         final accuracy……………………..1mk (maximum 5mks)
         Temparature of the solution of solid T= ±2 SV ( Note temperature to be given as whole number,1 decimal, or 2 decimal places otherwise penalize fully)
         (v)(I) Moles of acid T IN 25CM3
         Moles of NaOH = Ans in b(ii)×Average titre
                                        1000
         Mole ratio = 1: 1
         Moles of NaOH = Moles of acid T
         correct ans (penalize for strange value
         (v)(II) Moles of acid T in 100 CM3 of solution of acid T
         Correct answer in (v)(a)above ×100

                                            25
                                =correct ans
                  (V)(III) Solubility of the acid
                  RMM of acid T C7H6O2 = 122
                   Mass of T in 100cm3 of acid T = correct answer in (v) (II) above × 122  =correct ans
                 Solubility of the acid T = Correct answer above in g/100g water at temp in (b)(iv).
                                 (penalize ½ mk for missing units)

Question 2

complete table………………..1mk
decimal place used consistently ½ mk
accuracy ……………………………………. ½ mk
trend…………………………………………. ½ mk

3. 1. Graph
      Both correctly labelled axes with units ………. ½ mk
      Scale (over ¾ page) ……………………………………. ½ mk
      Plots (must be visible) ………………………………… 1 mk
      Shape (straight line thru origin) ………………….. 1 mk
   2. Time taken
      Showing on graph……………………………………….. ½ mk
      Working out ………………………………………………… ½ mk
   3. Rate of reaction is directly proportional to increase in concentration as there are more H+ IONS available to react with magnesium.
      OR
      Rate of reaction increases with increase in concentration as there are more H+ IONS available to react with magnesium.
4. Place a spatula end full of solid G in a boiling tube and add 100cm3 of distilled water while shaking. Divide the solution into four portions.
   

   Observation	Inference
   Dissolves(1/2 mk)forming a colourless solution (1/2 mk) (1mk)	Soluble salt (1/2 mk) Absence of Cu2+ ,Fe2+ ,Fe3+ (1/2mk )  ( 1 mk)

   
   
   1. to the first portion, add sodium hydroxide dropwise till in excess.
      

      Observation	Inference
      No white ppt formed ( ½ mk)	Probably Na+, K+, NH4+Present OR  Ca2+ , Mg2+ present  ( 1 mk)

      
      
      
   2. To the second portion, immerse a glass rod into the solution and place the wet rod into a non-luminous flame of a Bunsen burner.
      

      Observation	Inference
      Golden yellow flame observed  ( ½ mk)	Na+ present   ( ½ mk)

   3. To the third portion, add a few drops of barium nitrate solution followed by dilute nitric acid.
      

      Observation	Inference
      White ppt (1/2 mk) formed which dissolves when dilute nitric acid is added (1/2 mk) forming a colourless solution	CO32-  , SO32- present   ( 1  mk)

b. You are provided with solid H which is to be dissolved in about 6cm³ of distilled water. Divide the resulting solution into 5 portions carry out the tests described and note down
your observations and inferences.

1. To the 1st portion add 2-3 drops of acidified potassium dichromate (vi).
   

   Observation	Inference
   Orange colour of potassium dichromate (VI) persist (1 mk)	R-OH Absent (1/2 mk)

2. To the second portion add 2-3 drops of acidified potassium manganate (VII)
   

   Observation	Inference
   Acidified potassium manganate (VII) changes colour from purple to colourless.   (1 mk)	=C=C= or –C=C- present(1 mk)

3. To the third portion add 2-3 drops of bromine water.
   

   Observation	Inference
   Yellow colour of bromine water is decolourised(1 mk)	=C=C= or –C=C- present(1 mk

4. Place a quarter spatula full of sodium hydrogen carbonate in a dry test tube. Add the 4th portion to the NaHCO3 in the test tube.
   

   Observation	Inference
   Effervescence/ Bubbles formed(1/2) mk)	R-COOH/ -COOH present( ½  mk)