Chemistry Paper 3 Questions, Answers and Confidential - MECS Pre Mock Exams 2023

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Physics Paper 1 Questions and Answers - MECS Pre Mock Exams 2023

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INSTRUCTIONS TO CANDIDATES

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 read the question paper and make sure you have all the chemicals and apparatus that you may need.
Mathematical tables and silent electronic calculators may be used
ALL workings MUST be clearly shown where necessary.

FOR EXAMINER’SUSE ONLY

QUESTION	MAXIMUM SCORE	CANDIDATE’S SCORE
1	23
2	10
3	7
TOTAL SCORE	40

QUESTIONS

1. You are provided with;
  - 0.3 g of metal A
  - 70cm³ of 1.0 M hydrochloric acid solution labelled as solution B.
  - 100cm³ of 0.1 M sodium hydroxide labeled solution C.
  - Phenolphthalein indicator.

You are required to determine the relative atomic mass of metal A

Procedure I

Using a burette measure 50cm³ of solution B into a 250ml beaker.
Add the whole amount of solid A provided into the beaker containing 50.0cm³ of solution B and swirl carefully until ALL the solid reacts completely.
Transfer the mixture left in the beaker after the reaction into a 250ml volumetric flask. Rinse the beaker with distilled water and transfer all the rinsing’s into the volumetric flask. Make up the volume of the solution in the volumetric flask up to the mark with distilled water, shake well and label the solution D.
Fill a clean burette with solution D.
Pipette 25cm³ of solution C into a 250ml conical flask, add 3 drops of phenolphthalein indicator solution and titrate against solution D from the burette. Record your results in table I below.
Repeat the titration TWO more times to complete table I.

Table I (4marks)

	I	II	III
Final burette reading, cm³
Initial burette reading, cm³
Volume of solution D used, cm³

Calculate the average volume of solution D used, cm³ (1mark)
Calculate
1. Calculate the number of moles of HCI in 50.0 cm³ of solution B. (1mark)
2. Determine the number of moles of NaOH in 25.0 cm³ of solution C. (1mark)
3. Determine the number of moles of HCl in the average volume of solution D used in the titration. (1mark)
Calculate;
1. The number of moles of HCl in 250cm³ of HCI solution D. (1mark)
2. The number of moles of HCl that reacted with metal A. (1 mark)
Given that metal A forms a divalent cation,
1. Determine the moles of metal A that reacted with hydrochloric acid. (1mark)
2. Determine the Relative atomic mass of metal A. (1 mark)

1. (b) You are provided with

2.0g of solid E in a dry boiling tube.
A thermometer
Distilled water
Hot water bath

You are required to determine the temperatures at which solutions of known concentrations of compound K becomes saturated and plot a solubility curve.
Procedure II

Using a burette, add 5.0 cm³ of distilled water into the boiling tube with solid E.
Place the boiling tube into a water bath and warm it while stirring with a thermometer until all the solid dissolves.
Remove the boiling tube from the hot water and allow the content to cool slowly while stirring with the thermometer. NOTE the temperature at which crystals start to appear and record this temperature in table II.
Add a further 2.0 cm³ of distilled water from the burette into the boiling tube containing the mixture and repeat steps (c) and (d) above.
Repeat the procedure (e) above until the volume of water added is 15.0 cm³.
Complete table II by calculating the solubility of compound E in water at different temperatures.

Table II (5 marks)

Total volume of water added (cm³)	Temperature at which crystals first appear. (^oC)	Solubility of substance E in g/100g water.
5.0
7.0
9.0
11.0
13.0

On the grid provided plot a graph of solubility of compound E (vertical axis) against temperature. (3 marks)

graph aytfdyta

From the graph determine the temperature at which the solubility of E in water at 35.0 ºC/100g of water (1mark)
Determine the mass of crystals formed when a hot saturated solution of compound E is cooled from 70ºC to 45ºC (1 mark)
What is the relationship between the solubility of compound E and change in temperature (1mk)

2. You are provided with solid F. Carry out the tests below and record your observations and inferences in the spaces provided.
Place all the solid F provided in a boiling tube. Add 10 cm³ of distilled water and shake well. Divide the resulting solution into five portions of 2 cm3 each.

Observations

Inferences

(1 mark)

(1mark)

To the first portion, add 2 drops of barium nitrate solution followed by 2 cm³ of dilute nitric (V) acid.

Observations

Inferences

(1mark)

(½mark)

To the second portion, add 2-3 drops of lead (II) nitrate solution followed by 2 cm³ of dilute nitric (V) acid.

Observations

Inferences

(1mark)

(½mark)

To the third portion add about 2-3 drops of sodium sulphate.

Observations

Inferences

(1mark)

(1 mark)

To the 4th portion add sodium hydroxide dropwise until in excess.

Observations

Inferences

(1mark)

(½ mark)

To the 5th portion add ammonia solution dropwise until in excess.

Observations

Inferences

(1mark)

(½ mark)

3. You are provided with solid G. Carry out the tests below and record your observations and inferences in the spaces provided.

Scoop a little of solid G using a metallic spatula and burn it in a Bunsen burner flame.

Observations

Inferences

(1mark)

To the remaining portion of solid G, add about 6cm3 of distilled water and shake. Divide the mixture into two portions.

Observations

Inferences

(1mark)

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

Observations

Inferences

(1mark)

To the second portion add solid sodium carbonate provided.

Observations

Inferences

(½mark)

MARKING SCHEME

TABLE I……………..Total 5 mks distributed as below
- Complete table ----------------------------1mk
  Complete table with 3 titrations ------1mk
  Penalties
  Unrealistic titre values i.e. values below 1cm3 or hundreds
  Burette readings beyond 50cm3 unless explained
  Inverted table
  Wrong arithmetic
  N/B: Penalize ½ mk each for a maximum of ½ mk
- Decimals ----------1mk(Tied to the 1st and 2nd rows only)
  Should be 1 decimal place or 2 decimal places used consistently otherwise penalise fully.
  The 2nd decimal place should either be a 0 or 5
- Accuracy ------------1mk
  If any titre value within ± 0.1 of the school value ---------------------------------award 1mk
  If any titre within ± 0.2 of the school value ½ mk
  If none of the titre values is within ± 0.2 of school value (s.v)-----------award 0mk
- Principles of Averaging -------------1mk
  Values to be averaged MUST be shown and MUST be within ± 0.2 of each other
  Conditions
  If 3 consistent values are averaged -----------1mk
  3 titrations done only 2 are possible and averaged ---1mk
  3 consistent values but only 2 are averaged award 0mk
  3 inconsistent values are average award 0mk
  Penalties
  Penalise ½ mk for arithmetic error in answer outside ± 2 units in the 2nddec. Place.
  Penalise ½ mk for NO WORKING shown but the answer is correct.
  Accept rounding off to the 2nddec.places otherwise penalise ½ mk if answer is rounded off to 1stdec. place unless values divide exactly to 1 dec. place
- Final answer --------------------1mk
  (Compared to school value (S.V) Tied to correct average titre)
  If within ± 0.1 of S.V --------- award 1mk
  If within ± 0.2 of S.V -------- award ½mk
  If beyond ± 0.2 of S.V--------- award 0mk
1. Moles of HCI in 50cm3 of solution B
  = (1.0 ×50 )/1000㇢½=0.05 moles㇢½
2. Moles of NaOH in 25cm³ of solution C
  = (0.1×25)/1000㇢½=0.0025 moles㇢½
3. Moles of HCI in average volume of solution D used
  Mole ratio of NaOH: HCI = 1: 1㇢½
  Therefore moles of HCI = moles of NaOH = 0.0025 moles. ㇢½
1. Moles of HCI in 250 cm³ of solution D.
  (0.0025 ×250 )/(average volume )㇢½=correct answer.㇢½
2. Moles of HCI that reacted with metal A.
  = 0.05 - answer c (i) above ㇢½
  = correct answer ㇢½
1. Moles of A reacted:
  A + 2HCI → ACI2 + H2
  = answer c (ii) above ÷ 2 ㇢½
  = correct answer ㇢½
2. Relative atomic mass of metal A
  = (0.3 )/(answer d(i) above )㇢½
  
  =correct answer.㇢½ (Reject answer in decimal places)

PROCEDURE II
TABLE II ……………..Total 5 mks distributed as below

Temperature column------------------------------------------------------------------------2½ mks)
Completely filled table ……….. 1 mk
decimals ………………………..1 mk
(Accept whole numbers or 1 d.p (.5) or 2 d.p (.25, 50 or .75)
Trend -----------------------------------½ mk
(Should be a continuous decrease in temperature with increase in volume of water added)
Solubility values------------------------------------------------------------------------------- 2½ mks)
(Award ½ mk for each correctly worked out value of solubility)
NOTE:
Penalize ½ mk each for wrongly worked out value of solubility up to a maximum of 2½ mks
Graph.
Labeling of axes (both) ----------------------------------------------------------------------------- ½ mk)
Penalise ½ mk for wrong units used in any of the axis otherwise ignore if units not given.
Penalise ½ mk for inverted axis
Accept for ½ mk if no units shown on labeling.
Scale ----------------------------------------------------------------------------------------------------½mk)
Area covered by actual plots must be at least half of the big squares (y-axis) and half of the big squares ( x – axis)otherwise give zero.
Scale used must be consistent on both axes, otherwise penalise fully.
Plotting -------------------------------------------------------------------------------------------------1mk)
Accept 5 points correctly plotted for 1mk
If 3 or 4 points are correctly plotted award ½ mk
If less than 3 are correctly plotted award 0mk
Accept correct plots even if the axes are interchanged.
Curve -----------------------------------------------------------------------------------------------------1mk)
1. Award ½mk for extrapolation
  Award another ½ mark for correct reading.
  Penalize ½ mark for wrong units or no units given.
2. Correct solubility reading at 70ºC – correct solubility reading at 45ºC ㇢½
  = correct answer ㇢½ (Penalize ½ mark for wrong units or no units given)
3. Solubility of compound E increases with increase in temperature.

QUESTION 2

Observations

Inferences

Solid dissolves ㇢½ to form a colourless solution㇢½

Salt is soluble ㇢1

Accept for ½ mk – Cu²⁺, Fe²⁺, Fe³⁺ absent

Observations

Inferences

White precipitate ㇢½ that does not dissolve ㇢½ in nitric (v) acid.

SO₄^2- present ㇢½

(Penalize fully for any contradictory ion/s)

Observations

Inferences

White precipitate ㇢½ that does not dissolve ㇢½ in nitric (v) acid.

SO₄^2- confirmed㇢½

(Penalize fully for any contradictory ion given)

Observations

Inferences

No white precipitate㇢1

Accept for ½ mk

- No observable change

- Solution remains colourless

Reject

No change.

No reaction.

i.e award zero mk.

K⁺, Na⁺ , Mg²⁺, Zn²⁺, Al³⁺ present

All five given -award 1 mk.

Any 3 correct – award ½ mk

Less than 3 given -award 0 mk.

Penalize ½ mk each for any contradictory ion up to a maximium of 1 mk.

Accept for ½ mk Ca²⁺,Ba²⁺,Pb²⁺ absent s

Observations	Inferences
White precipitate㇢½ formed insoluble㇢½ in excess	Mg²⁺ present ㇢½

(vi)

Observations	Inferences
White precipitate㇢½ formed insoluble㇢½ in excess	Mg²⁺ present ㇢½

QUESTION 3

Burning a little solid G using a Bunsen burner flame.

Observations

Inferences

Burns with a yellow㇢½ sooty ㇢½ flame – accept burns with a luminous flame for ㇢1

=C=C= / -C≡C- present㇢1

(for either)

Add about 6cm³ of distilled water and shake.

Observations	Inferences
Solid dissolves to form a colourless solution. ㇢½	Solid is polar/ polar compound. ㇢½

To the 1^st portion add acidified potassium manganate (VII) solution.

Observations

Inferences

Purple acidified potassium manganate (VII) is decolourised. ㇢1 / changes to colourless

=C=C= / -C≡C- present ㇢½ (for either)

R-OH ㇢½

(d) To the 2^nd portion add sodium hydrogen carbonate

Observations

Inferences

Effervescence /bubbles /fizzing㇢½

Reject: fissing/fizzling/hissing.

R-COOH㇢½ or / H⁺/H₃O⁺

- Reject fully any contradictory functional group. i.e. award 0 mk.

CONFIDENTIAL

The information contained in this confidential is to enable the head of institution and the teacher in charge of chemistry to make adequate preparations for the practical examination.

Note: No one else should have access to this information either directly or indirectly

INSTRUCTIONS

In addition to the laboratory fittings each candidate requires the following: