Chemistry Paper 3 Questions, Answers and Confidential - Asumbi Girls Highschool Pre-Mock Exams May-June 2022

INSTRUCTIONS TO CANDIDATES

• Write your name and index number in the spaces provided above.
• Sign and write the date of examination in the spaces provided.
• Answer ALL the questions in the spaces provided in the question paper
• Mathematical tables and electronic calculators may be used.
• All working MUST be clearly shown where necessary.

FOR EXAMINER'S USE ONLY

Question	Maximum marks	Candidate's score
1	22
2	12
3	6
Total score	40

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QUESTIONS

1.                      
   1. You are provided with
      • 2.0M sodium hydroxide solution labelled solution B
      • Solution C containing 12.25 g per litre of a mineral acid C
        
        You are required to
        1. Prepare a dilute solution of sodium hydroxide, solution B.
        2. Determine the
           Relative Formula mass of the acid C
           Molar Enthalpy change of reaction between acid C and sodium hydroxide solution B.

Procedure 1.
Using a pipette and a pipette filler place 25.0cm³ of solution B in a 250.0ml volumetric flask. Add to
it about 150cm³ of distilled water. Shake well. Add more distilled water to make upto the mark. Label this solution D.

Fill a burette with solution C. Using a clean pipette and a pipette filler, place 25.0cm³ of solution D into a 250ml conical flask. Add two drops of phenolphthalein indicator and titrate with solution C. Record your results in table 1. Repeat the titration two more times and complete the table. (4 marks)

	1	2	3
Final burette reading (cm³)
Initial burette reading (cm³)
Volume of solution C used (cm³)

Calculate:

1. Average volume of solution C used. (1 mark)
2. Moles of solution D used. (2 marks)
3. Concentration in moles per litre of acid in solution C given that the number of moles of acid C used are half the moles of D used. (2 marks)
4. Relative formula mass (RFM) of solution C. (1 mark)

b) PROCEDURE II.

1. Using a clean burette, place 5.0cm³ of solution C into each of six (6) test-tubes.
2. Using a 100ml measuring cylinder, place 20cm³ of solution D, sodium hydroxide solution in a 100ml plastic beaker. Measure the temperature of solution D and record it in table 2 below.
3. To solution D in the beaker, add acid C, solution C from one of the test-tubes. Stir the mixture with the thermometer and record in Table 2, the maximum temperature reached. Continue with step (iv) IMMEDIATE
4. Add the acid C, solution C from another test-tube to the mixture obtained in (iii) above, stir and record the maximum temperature reached in Table 2. Continue adding the acid C, solution C from each of the other four test-tubes, stirring the mixture and recording the maximum temperature each time and complete Table 2.

TABLE 2 (4 marks)

Volume of solution C acid C added (cm³)	0	5	10	15	20	25	30
Maximum temperature (°C)

c) On the grid provided, plot a graph of temperature (vertical axis) against volume of acid C solution C added. (3 marks)

d) Using the graph

1. Determine the volume of solution C which gave the maximum change in temperature. (1 mark)
2. Determine the temperature change for the reaction. (1 mark) 

e) Using your answer in parts d (i) and d(ii), calculate the molar enthalpy change of the neutralisation reaction between acid C and sodium hydroxide solution.
(Heat capacity = 4.2J g-1 k-1; density of the mixture = 1.0gcm-3) (3 marks)
2. 

1. You are provided with solid F carry out the following tests write your observations and inferences in the spaces provided.

i) Place a half spatula and full of solid F in a dry test tube and heat strongly. Test the gases produced with litmus paper

Observations	Inferences
(1mark)	(1 mark)

ii) Place the remaining solid F in a boiling tube and add 10cm³ of distilled water. Divide the resulting solution into five portions

Observations	Inferences
(1 mark)	(1 mark)

iii) To the first portion, add sodium hydroxide solution drop wise until in excess.

Observations	Inferences
(1mark)	( 1 mark)

iv) To the second portion, add 5 drops of the hydrogen peroxide and then add ammonia solution drop wise until in excess.

Observations	Inferences
(1 ½ mark)	( ½ mark)

v) To the third portion, add three drops of acidified lead (II) nitrate solution.

Observations	Inferences
( 1 mark)	(1mark)

vi) To the fourth portion, add three drops of barium nitrate solution.

Observations	Inferences
( 1 mark)	( 1 mark)

3.   You are provided with an organic liquid P. Carry out the following tests and record you observations and inferences in the space provided.
a)   Place about 4cm³ of liquid P in a boiling tube. Add to it 10cm³ of distilled water and shake well.
Label this solution G.

Observations	Inferences
( 1 mark)	(1mark)

b)   Place 2cm³ of solution G in a test-tube. Add to it solid sodium hydrogen carbonate provided.

Observations	Inferences
( 1 mark)	(1mark)

c)   To a second 2cm³ portion of solution G in a test-tube, add 2 to 3 drops of acidified potassium dichromate (VI) and warm.

Observations	Inferences
(1 mark)	(1mark)

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MARKING SCHEME

1. PROCEDURE 1
   Table 1 . . . . . .(5 marks) Complete table      . . . . . 1 mark
   Complete table with 3 titration done - 1 mk
   Incomplete table with 2 titrations done - ½mk
   Incomplete table with 1 titration done - 0 mk

Penalties - Penalise ½ mk ONCE for

1. Wrong arithmetic / subtraction
2. Inverted table.
3. Burette    readings    beyond    50cm³    unless explained.
4. Unrealistic titre values e.g. below 1.00cm³ or in hundreds. 

•  If two d.ps used, the second decimal must be either a zero of five.

1. If   three   consistent   titrations   done   and averaged . . . 1 mark
2. If three titrations done but only two are consistent and averaged . . .1 mark
3. If    two    titrations    done,    consistent    and averaged . . . 1 mark
4. If three titrations are done, inconsistent but averaged = 0 mark

1. Penalise ½ mk if no working is shown but answer given is correct.
   Penalise fully if no working is shown and the answer is wrong.
2. Accept  rounding  off  or  trancating  of  the answer to 2d.p e.g. 25.666 ~ 25.66 or 25.67 otherwis4e penalise ½ mk for rounding off to 1 d.p
   Accept the answer if it works out exactly to a whole no. or to 1.d.p and credit fully.

Final accuracy . . . 1 mark
Tied to correct averaged titre.
Compare the candidates correct averaged titre to the school value.
If within ± 0.10 cm³ of S.V. . . . 1 mark
If not within ± 0.10 cm³ of S.V but withint ±
0.20cm³ of S.V award . . . .½ mark
If beyond (outside) ± 0.20cm³ of S.V .0 mk

Calculations
Moles of solution D used

Moles of B used = 25.2 = 0.05 moles
                                2
= 1000 = 0.05 = 0.2 m
   250

Hence moles of solution D used
= 0.005 moles
Molarity of solution C

Conditions.

1. Ans must not have any unit otherwise penalise ½ mk for g or any other unit.
2. Accept R.FM as :
   88 ≤ RFM ≤ 108

Procedure II
TABLE

1. Complete table . . . . 1 mark
   Complete table with seven (7) readings and realistic values
   Use of decimal . . . (1 mark)
   1. If one decimal place is used, it can either be 0 or 5.
   2. If 2 d.p are used then they must be 0.00, 0.25, 0.50 or 0.75
   3. Decimal points must be used consistently as either whole numbers or 1d.p

Accuracy . . . .1 mark
Compare the candidates constant temperature readings with school value (S.V) S.V is the teachers reading at 0. If within ± 2.0°C of S.V award 1 mark.

Trend . . . 1 mark
Conditions.

1. Award 1 mark for a continuous rise followed by a constant then a drop
   Graph . . . . 3 marks
   Award a total of 3 marks distributed as follows.

i). Labelling of axes . . . . ½ mark
If both axes are correctly labelled.

Penalties.
Penalise fully for inverted axes.
Penalise fully for wrong units used. Otherwise ignore if units are omitted.
Penalise fully (award 0) if only one axes is correctly labelled.

Scale . . . . ½ mark
Conditions.

1. Area covered by plots should be at least ½ the grid provided i.e .7 big squares verti and 8 big square horizontally.
2. Scale interval must be consistent on each axis.
3. Scale chosen must be able to accommodate all the readings whether plotted or not.

Plotting . . . 1 mark
If 7 or 6 points are correctly plotted . . 1mk
If 4 or 5 points correctly plotted . . .½ mk
If less than 4 points correctly plotted . . . 0mk

NB

1. Accept correct plots even if the axes are inverted and award accordingly.
2. If scale intervals are inconsistent credit correct plots. If any, within the 1st scale interval and treat all other plots as wrong.
3. Mark all the plots on the graph with either a tick  or a cross (×)

In addition to the apparatus  and reagents found in a Chemistry laboratory each candidate will require the following :