Chemistry Paper 3 Questions and Answers - Cekenas Mock Exams 2022

MARKING SCHEME

QUESTIONS

You are provided with the following
Sodium hydroxide solution A
0.1M HCl labelled B
An alkanoic acid labelled C
You are required to

Standardize solution A
Determine the mole ratio of reaction between sodium hydroxide and the alkanoic acid
Calculate the molarity of solution C

Determine the molar enthalpy of neutralisation of the alkanoic acid and sodium hydroxide
PROCEDURE 1
Using the pipette, place 25cm³ of solution A into a 250ml volumetric flask .Add distilled water as you shake up to the 250ml mark .Label the solution formed as D.
Fill the burette with solution B .Rinse the pipette and use it to transfer 25cm³ of solution D into a conical flask. Add 3 drops of phenolphthalein indicator and carry out titration .Repeat the procedure twice to obtain concordant values .Record the results below

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

(4mk)

Determine the average titre volume (1mk)
Calculate the number of moles of
1. HCl that reacted (1mk)
2. NaOH in the 25cm³ of solution D ( 1/2mk)
3. Molarity of NaOH solution A (1mk)

PROCEDURE 2
Rinse the burette and fill it with solution C .Run 16cm³ of solution C into a 100ml plastic beaker .Using a thermometer measure its temperature and record it in the table 2 below as initial .Add 4cm³ of solution A using a syringe .Stir with the thermometer immediately and note the highest temperature reached .Record it in the table as final temperature .Repeat the procedure with other volumes of A and C as shown in the table and complete it . Rinse the beaker and the thermometer after each experiment

On the grid below, plot the graph of change in temperature against volume of sodium hydroxide solution A (3mks)
From your graph determine,
1. The volume of sodium hydroxide solution A required to neutralize the alkanoic acid (1/2mk)
2. Highest change in temperature DT (1/2mk)
Determine the volume of the alkanoic acid solution C, used in the neutralization (1/2mk)
Calculate the;
1. Concentration of the alkanoic acid solution C, given that the ratio of volume of alkali to the volume of acid is also the mole ratio(2mks)
2. Molar enthalpy of neutralisation of the acid by sodium hydroxide(2mks)
  (C=4.2J/g/k, density of solution =1g/cm³)

You are provided with solid P .Carry out the following tests and write your observations and inferences in the spaces provided
1. Place all the solid P in a boiling tube .Add about 10cm³ of distilled water and shake the mixture thoroughly .Filter the mixture into another boiling tube .Retain the filtrate for use in the test 2 (b) below and dry the residue using a filter paper
  1. Transfer about half of the residue in a dry test tube .Heat the residue strongly and test any gas produced using a burning splint
    
    Observations Inferences
  2. Place the rest of the residue in a dry test tube .Add 2cm³ of 2M hydrochloric acid .Retain the mixture for test (iii) below
    
    Observations Inferences
  3. To the solution formed in (ii) above, add aqueous ammonia drop wise till in excess
    
    Observations Inferences
2. 1. To above 2cm³ of the filtrate obtained in a) above add aqueous ammonia drop wise until in excess
    
    Observations Inferences
  2. To about 2cm³ of the filtrate, add about 2cm³ of 2M hydrochloric acid
    
    Observations Inferences
  3. To 2cm³ of filtrate add two drops of barium nitrate solution(2mks)
    
    Observations Inferences
You are provided with solid E carry out the following test and record your observations and inferences in the spaces provided
1. Place about one half of solid E in a dry test tube .Retain the other half for use in (b) below .Add all of the absolute ethanol provided to solid E in a test tube shake the mixture
  Divide the mixture formed into two portions
  1. Determine the pH of the first portions
    
    Observations Inferences
  2. To the second portion, add about one half of sodium hydrogen carbonate provided
    
    Observations Inferences
2. Place the remaining solid E in a boiling tube 10cm³ of distilled water and shake. Boil the mixture and divide it into 3 portions while still warm
  1. To the first portion add the remaining sodium hydrogen
    
    Observations Inferences
  2. The second portion add three drops of acidified K₂Cr₂0₇ (1mks)
    
    Observations Inferences
  3. To the third portion add 5 drops of Bromine water(1mk)
    
    Observations Inferences

CONFIDENTIAL

Solution A-120cm³
Solution C -60cm³
5cm³ of absolute ethanol
About 0.5gms NaHC₃
About 0.2g solid E
About 1g solid P
Pipette
Pipette filler
250ml volumetric flask
Means of labelling
Burette
Filter funnel
Conical flasks – 2pcs
100 ml plastic beaker
Thermometer
20ml syringe
Boiling tube -2pcs
Filter paper -2pcs
6 test tubes in a rack
Wooden splint
Distilled water in wash bottle
Metallic spatula
Test tube holder

Access to;

Phenolphthalein indicator
Source of heat
2M NH₃
2M HCl
Ba(NO₃)₂
Universal indicator and pH chart

Preparation of solutions

Solution A -0.8 M NaOH
Solution C -1M H₂SO₄
Solid P –mixture of ZnCO₃ and ZnSO₄ in the ratio 2:1
Solid E is 0.2 grams Maleic acid

Volume of solution (cm³)

Volume of solution A (cm³)

Final temperature (ºC)

Initial temperature (ºC)

Change in temperature

2. 1. HCl that reacted (1mk)
    = correct answer A
  2. iNaOH in the 25cm3 of solution D( 1/2MK)
    Correct answer (i) above
  3. Molarity of NaOH solution A (1MK)
    Ans in (ii) → 25cm³? = 250cm³
    = Ans B
    Ans B x 1000
    25
    = correct ans
4. LA =1/2
  Scale =1/2
  P=1
  Line=1
5. 1. (To be read from the correct graph)
  2. (To be read from a correct graph)
6. 20 –Ans in (i) above
7. 1. vol of NaOH ;Acid
    2 :1
    Moles of sodium hydroxide
    Ans in (e) (ii) above x Ans in b(iii)
    1000
    =ans in c
    Moles of acid = Ans C/2
    Ans C x 1000
    2
    Ans in F
    =Correct answer
  2. Heat change for reaction 20 x 4.2 x Ans in e (ii) above
    = Correct answer (K) in joules
    Moles of acid = Ans in (F) above x correct Ans in g (i) above
    1000
    = Ans M
    =Correct Ans K /Ans M = - Correct Ans J/mole

Observations	Inferences
a)i)Effervescence ,burning splint extinguished Residue, yellow hot white when cold ii)Effervescence/ bubbles of a colourless gas iii)White precipitate soluble in excess b)i) White precipitate soluble in excess ii) No effervescence No white precipitate iii) White precipitate	CO₃^2- Zn²⁺ CO₃^2- Zn²⁺Zn²⁺ CO₃^2-, SO₃^2- absent Pb²⁺ Ag+ absent SO₄^2-

Observations	Inferences
a)Soluble i)pH7 ii)No effervescence/bubbles b)i)Effervescence ii)Orange colours of H+/ K₂Cr₂O₇ changes to green iii)Yellow bromine water decolourises/ changes to colourless	polar neutral RCOOH/ /H₃O⁺/H⁺ Absent RCOOH/H₃O⁺ R-OH