Chemistry Paper 3 - 2021 KCSE Prediction Questions and Answers Set 1

Instructions to Candidates

Answer All questions in this question paper
You are Not allowed to start working with the apparatus for the first 15 minutes of 2¼ hours. This is to ensure you read the question paper and make sure you have all chemicals and apparatus you may need.
All working Must be cleary shown where necessary
Silent calculator may be used
Candidate to answer questions in English.

You are provided with;
- Alkaline solution labeled B.
- 1.0 M hydrochloric acid labeled A.
- Dilute dibasic acid labeled Q.
  You are required to standardize B with A and obtain molar heat of neutralization of B using dilute dibasic acid.
  PROCEDURE I
  1. Place solution A in a clean 50cm³
  2. Using 25ml pipette, place 25cm³ of solution B in a conical flask.
  3. Add 2 drops of phenolphthalein indicator and titrate with solution A.
  4. Record your results in table I repeat two times and complete the table. (4mks)
    
    I
    
    II
    
    III
    
    Final burette
    
    Initial burette reading cm³
    
    Volume of solution A used cm³
    
    Calculate the;
    1. Average volume of solution A used. (1mk)
    2. Moles of A that reacted with 25 cm³ of solution B. (1mk)
    3. Moles of solution B that reacted with 25cm³ of solution A given that the cation of alkaline solution has a charge + 1. (2mk)
    4. Concentration of B in moles per litre. (1mk)
- PROCEDURE II
  1. Using a measuring cylinder, measure 50 cm³ of solution B in a 250mls plastic beaker.
  2. Measure the temperature of B and record in table II below.
  3. Using a measuring cylinder, measure 10cm³ of Q and add into the plastic containing 50 cm3 of solution B. stir the mixture using a thermometer and record the highest temperature in the table II below.
  4. Repeat this procedure by adding 10cm³ of solution Q for five times while recording the highest temperature formed. Complete table II below. (4mks)
    
    Total volume of solution Q in cm³
    
    0
    
    10
    
    20
    
    30
    
    40
    
    50
    
    Volume of solution B in cm³
    
    50
    
    50
    
    50
    
    50
    
    50
    
    50
    
    Highest temperature change
    1. On the grid provided plot a graph of temperature (y-axis) against volume of solution Q ( x- axis. (3mks)
    2. From the graph. Find;
      1. Volume of the solution Q required to neutralize 50 cm ³ of solution B. (1mk)
      2. Highest temperature change. (1mk)
    3. Determine the molar heat of neutralization of solution B.
      (Density = 1g/cm³, specific heat capacity = 4.2 kj/kg/k. molar mass of B = x grams ) (2mks)

You are provided with solid Z carry out the test below and record your observation and inferences in the spaces provided.
1. Place about a third of solid Y in a dry test-tube and heat strongly.
  Observations (1mk)
  Inference (1mk)
2. Put a spatulaful of Z in a boiling tube. Dissolve with distilled water and divide into four portions.
  Observations (½ mk)
  Inferences (½ mk)
3. To the first portion add sodium hydroxide droo wise until excess.
  Observation. (½ mk)
  Inference. (½ mk)
4. To the second portion add ammonia solution drop wise until excess.
  Observation (½ mk)
  Inference (½ mk)
5. To the third portion is suspected to contain Lead II ions. Give the test and observation that are to be made to confirm presence of Lead II ions.
  Test (1mk)
  Inference (1mk)
6. Carry out the test above to confirm the presence of Lead II ions.
  Test (1mk)
  Inference (1mk)
7. To the fourth portion add few drops of Barium Nitrate solution.
  Observation (½ mk)
  Inference (½ mk)

You are provided with slid S. Carry out the tests below and write your observations and inferences.
1. Using a clean metallic spatula. Heat solid P in a non luminous flame.
  Observation (½ mk)
  Inference (½ mk)
2. Dissolve remaining solid S in 10cm3 of distilled water in a boiling tube. Pour solution in four portions.
  Observation (1mk)
  Inference (1mk)
3. To the first portion and 3 drops of acidified Potassium permanganate.
  Observation (½ mk)
  Inference (½ mk)
4. To the second portion add a few drops of solution S and few drops of solution x. warm the solution.
  Observation (1mk)
  Inference (1mk)
5. To the third portion add spatula full of sodium carbonate provided.
  Observation (1mk)
  Inference (1mk)
6. To the fourth portion, determine the PH of the solution using universal indicator paper.
  Observation (1mk)
  Inferences. (1mk)

MARKING SCHEME

Table 1

	I	II	III
Final burette reading cm³	25.0	25.0	25.0
Initial burette reading cm³	0.0	0.0	0.0
Volume of solution N used	25.0	25.0	25.0

The marks are to be distributed as follows.

Complete table (1mk)
Incomplete table with 2 titration’s done a ward (1mk)
Incomplete table with 1 titration done award( ½ mk)

Penalize

Wrong arithmetic
Inverted table
Burette reading above 50 unless explained.
Unrealistic titre value i.e values in number in hundreds or below 1.0cm³Penalize ½ mark for each to a maximum of ½ mark each to a maximum of ½ mark once.
Use of decimals(1mk)
1. Accept only 1 or 2 decimal places used contently otherwise penalize FULLY and award 0 mark.
2. If the two decimal places are used the 2^nd decimal place be either “0” or “5” otherwise penalize fully.
  Compare the candidate titre value with the teachers value. (1mk)

Conditions

If at least one of the titre value is within 1cm3 of the teachers value awrd.... (1mk)
If no value is within 1 of the teachers value but at least one is within 0.2 of teachers value award ........ ( 1mk)

Conditions

If 3 consistent titration are done and only two consistent and average award ................ (1mk)
If 3 titrations done and only two consistent and average award...................... (1mk)
If only two titrations’ done are consistently averaged award.............................(1mk)
If three titres are possible but only two averaged award................................... (0mk)
If 3 inconsistent titres are averaged award.............................(0mk)
If only 1 titration is done award............................(0mk)
Final answers..................................................................................................................... (1mk)

Compare the candidate’s correct average award.

If within 1 of the teacher value award.......................................................... (1mk)
If no within 1 of the teachers value but within 0.2 award....................... ( ½ mk)
If beyond 2 of the teacher’s value award...................................................... (0mk)
Calculation
Moles of A used.
Mol = (moles )/(vol in litres)
Moles = mol x ^(vol)/₁₀₀₀
= 1 x ²⁵/₁₀₀₀
= 0.025 moles.
Moles ratio 1:1
Moles of B = 0.025 moles.
Mol = ^(moles)/_{(vol in litres)}
= ^0.025/₂₅
=1000
= 1m.

PROCEDURE 2:

Table II condition to apply
Trend in temperature
Graph: condition to apply.
Answers

1. Observation
  A colourless gas that turns moist red litmus paper blue.
  Moist blue litmus paper remain blue.
  Droplets of colourless liquid on cooler parts of the test-tube.
  Inference
  Basic gas NH₄⁺Z-hydrated salt.
2. Observation
  Dissolve to form a solution.
  Inference
  Soluble salt.
3. Observation
  White ppt soluble in excess.
  Inference
  Zn ^2+,Pb^2+. Al ³⁺ present.
4. Observation
  White ppt insoluble in excess.
  Inference
  Pb 2+, Al³⁺ present.
5. Test
  Add solution of potassium iodide
  Observation
  Yellow ppt.
6. Observation
  No yellow ppt
  Inference
  Pb ²⁺ absent.
7. Observation
  White ppt.
  Inference
  SO₄^2-Present^.

1. observation
  - solid melts and burne with sooty flame.
  Inference.
  C = C - C = C- Unsaturation.
2. Observation
  - Solid dissolves to form colourless solution.
  Inference
  Polar compound.
3. Observation
  Purple colour of potassium permanganate decolourised.
  Inference
  C = C - C= C- , ROH.
4. Observation
  Pleasant smell.
  Inference
  RCOOH Present
5. Observation
  Effervescence / bubbles
  Inference
  RCOOH, HT present
6. Observation
  PH 4
  - Weak acid
  Inference .
  Weak acid.