Chemistry Paper 3 Questions And Answers with Confidentials - Form 3 Term 2 Opener Exams 2021

CHEMISTRY
PAPER 3
TERM 2 OPENER EXAM

INSTRUCTIONS

• Answer all the questions

1. You are provided with:
   • Solution X₁ which is 0.1 M NaOH
   • Solution X₂ which is a dibasic acid ,H₂Z
   • Phenolphthalein indicator.
     
     You are required to determine the concentration of the dibasic acid and the mass of the anion Z in the acid
     
     
   • Procedure
     Fill the burette with solution X2 .pipette 25cm3 of solution X1in to a conical flask and add two to three drops of phenolphthalein indicator .titrate X1 against X2 .repeat this procedure two more times and complete the table below .
     

     	I	II	III
     Final burette reading cm3
     Initial burette reading cm3
     Volume of solution X2 used cm3

     
     
     1. Determine the average volume of solution X2 used. 2mks
     2. Workout the number of moles of NaOH solution X1 used. 3mk
     3. Given that the equation for the reaction is :
        H₂Z_(aq) + 2NaOH_(aq) → Na₂Z_(aq) + 2H₂O_(l)
     4. Determine the number of moles of the dibasic acid H2Z used 3mks
     5. Work out the concentration of solution X2 in moles per dm3. 3mks
     6. Given that the concentration of solution X2 is 8.17g/dm3 determine the relative formula mass of H2Z. 3mks
     7. Find the molar mass of the anion Z^-2 in the acid H₂Z(H= 1.0) 3mks
        
        
2. You are provided with solid M .carry out the test below on M and fill the observation and inferences.
   

   Test	Observation	Inferences
   Observe solid M and describe its appearance	2mks	1mk
   Take half of solid M in a clean dry boiling tube and heat it gently	2mks	1mk
   Take the remaining solid M in to a boiling tube and add about 5cm3 of distilled water and shake divide the solution formed in to two portions.	2mks	1mk
   To the first potion add a few drops of ammonia solution and then in excess	2mks	1mk
   To the second portion add a few drops of barium chloride followed by a few drops of dilute hydrochloric acid.	2mks	1mk

   
   Hence write down the cation and anion in solid M(3mks)

MARKING SCHEME

1.     
   

   	I	II	III
   Final burette reading cm3	15.0	14.9	15.0
   Initial burette reading cm3	0.0	0.0	0.0
   Volume of solution X2 used cm3	15	14.9	15.0

   
   complete table
   Consistency in use of decimal places
   Accuracy( titre should be around 15.0cm³
   1.  ^{15.0 + 14.9}/₂= 14.95cm³ (correct principle of averageing)
   2. no. of moles of NaOH= ²⁵/₁₀₀ x 0.1=0.0025
   3. Mole ratio H₂Z: NaOH=1:2
      moles of H₂Z=^0.0025/₂ = 0.00125moles
   4. 14.95cm³ H₂Z contains 0.00125moles
      1000cm³ H₂Z contains ¹⁰⁰⁰/_14.95 x 0.00125 = 0.084m
   5. Molar mass=concentration/ molarity = 8.17/0.084=97
   6. (1x2)+Z=97
      2+Z=97
      Z=95
      
      
2.  You are provided with solid M .carry out the test below on M and fill the observation and inferences.

   Test	Observation	Inferences
   Observe solid M and describe its appearance	Blue crystals          2mks	M is likely to be a copper(ii) salt         1mk
   Take half of solid M in a clean dry boiling tube and heat it gently	White powder formed colourless liquid formed            2mks	M is a hydrated salte          1mk
   Take the remaining solid M in to a boiling tube and add about 5cm3 of distilled water and shake divide the solution formed in to two portions.	Solid dissolve forming pale-blue solution            2mks	M is likely to be a copper(ii) salt           1mk
   To the first potion add a few drops of ammonia solution and then in excess	Pale-blue precipitate formed which dissolves in excess to form deep-blue solution            2mks	Cu2+ present          1mk
   To the second portion add a few drops of barium chloride followed by a few drops of dilute hydrochloric acid.	White ppt formed insoluble in dilute HCl            2mks	Cu2+ present          1mk

   Cations - Cu²⁺ 
   Anions -  SO₄^2-