Chemistry Paper 3 Questions and Answers - Murang'a County Mocks 2020/2021

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CHEMISTY PAPER 3

PRACTICAL

KENYA CERTIFICATE OF SECONDARY EDUCATION (KCSE)

Instructions to candidates

Answer ALL the questions in section in the spaces provided.
ALL working MUST be clearly shown.

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 in indicator

You are required to:

Standardize solution A
Determine the rate of reaction between solution A and magnesium

PROCEDURE I

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

PROCEDURE II

Cut solid C into equal pieces, each 2cm long.
Using a burette, measure 12cm³ of solution A, into a clean boiling tube.
Drop one piece of solid C into the boiling tube containing solution A and start stopwatch immediately. Stop the stopwatch when all solid C has just reacted. Record your results in the table below.

Repeat steps (ii) and (iii) above using 10cm³, 8cm³, 6cm³ and 4cm³ of solution A. Top up each with distilled water to make 12cm³ of solution and complete the table below. (4 marks)

Volume of solution A (cm³)	Volume of distilled water (cm³)	Conceentration of solution A(mols/l)	Time(s)	¹/_t(s^-1)
12	0
10	2
8	4
6	6
4	8

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

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 (1 mark)
  
  Observation Inference
2. 1. Place the remaining solid Q in a boiling tube. Add 10cm³ of distilled water. Divide the solution into five portions. (2 marks)
    
    Observation Inference
  2. To the first portion, add aqueous lead (II) nitrate solution (1 mark)
    
    Observation Inference
  3. To the second portion add dilute nitric (V) acid, followed by barium nitrate solution (2marks)
    
    Observation Inference
  4. To the third portion add a few drops of sodium hydroxide until excess observation (2marks)
    
    Observation Inference
  5. To the fourth portion, add a few drops of aqueous ammonia until is excess. (2 marks)
    
    Observation Inference
  6. To the fifth portion, add a few drops of hydrochloric acid (1½ marks)
    Warm the contents.
    
    Observation Inference
You are provided with solid R. carry out the tests below and record your observations and inferences.
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. (1½ marks)
  
  Observation Inference
2. 1. Test the first portion with the universal indicator solution provided. (1½ marks)
    
    Observation Inference
  2. To the second portion, add a few drops of acidified potassium manganite (VII) solution (2 marks)
    
    Observation Inference
  3. To the third portion, add a few drops of bromine water (2 marks)
    
    Observation Inference
  4. To the fourth portion, add half spatula of sodium hydrogen carbonate (1 mark)
    
    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. (1 ½ Marks)
    
    Observation Inference

MARKING SCHEME

titration results
Marks distributed as follows:

Complete table (1mk)
- 3 titration 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)
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
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)
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

CALCULATIONS

(12.5+12.5+12.5) (½ mk) = 12.5cm³ (½ mk)
3
1. Moles in 250cm3 = ^0.5/₄₀=0.0125 moles (½ mk)
  Moles used
  = (0.0125 ×25) (½ mk)=0.00125 moles
  250
2. Moles of acid reacting = 0.00125(½ mk) mole ratio 1:1
  0.00125 =12.5
  ? = 250
  (0.0125 × 250) (½ mk)= 0.025 moles (½ mk)
  250
3. Molarity of solution A
  10cm³ = 0.25 moles
  1000 =?
  (1000 × 0.25) (½ mk) = 25M(½ mk)
  10

graph of inverse of time against concentration

PROCEDURE II
TABLE II

Volume of solution A (cm³)	Volume of distilled water (cm³)	Conceentration of solution A(mols/l)	Time(s)	¹/_t(s^-1)
12	0	2.5	25.5	0.0363
10	2	2.08	34.25	0.0292
8	4	1.67	45.45	0.0220
6	6	1.26	69.44	0.0144
4	8	0.83	120.01	0.0083

Complete table (4mks)
Decimal places (tied to 3rd and 5th column) (1mk)
Accuracy (tied to row (i) to (iv) (1mk)
Trend (gradual increase in time) (1 mk)

Questions
c(i) read from graph (1mk)
Value of ¹/_t (1mk)
Correct answer (½ Mk)

	Observation	Inference
(a)	- Colourless liquid condenses at coller parts of test tube - Gas evolved turns red litmus blue - A white residue remain Any 2 (1mk	- Hydrated salt - NH₄⁺ ions Any 2 correct ions award (1 mk)
(b)(i)	- Solid dissolves (½ mk) to form a colourless solution ( ½ mk)	Mg²⁺, Al³⁺, Zn²⁺ Present Any One Or Cu²⁺, Fe²⁺, Fe³⁺ absent ( ½ mk)
(ii)	white precipitate is formed ( ½ mk)	CO₃^2-,SO₄^2-.Cl-,SO₃^2- present Any two ions award ( ½ mk) Penalize the ( ½ mk) for any contradictory ion.
(iii)	A white precipitate form ( ½ mk)	SO^2-₄ ions confirmed ( ½ mk)
(iv)	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
(v)	White precipitate ( ½ mk) in excess	Pb²⁺, Al³⁺ present Both ions (1mk) One ion ( ½ mk)
(vi)	No white precipitate formed (1 mk)	Pb²⁺ absent or Al³⁺ present (½ mk)

	Observation	Inference
(a)	Dissolves to form (½mk) a colourless homogenous solution (½ mk)	Polar substance ( ½ mk)
b(i)	PH value 1 – 3 ( ½ mk)	Strong acid present ( ½ mk)
(ii)	Purple acidified KMnO₄decolorised (1 mk)	\| \| −C=C−, −C≡C− or −OH present
(iii)	Bromine water decolourised (1 mk)	\| \| −C=C−, −C≡C− or −OH present All three (1mk) Two only (½ mk)
(iv)	Effervescence ( ½ mk)	R − C = O or H⁺ present \| OH
v	Sweet smelling compound formed (1 mk)	R − C = O present \| OH