Chemistry Paper 3 Questions and Answers - Form 3 End Term 1 Exams

Answer all the questions in the spaces provided.

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 X₁ in 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. 3mks
     3. Given that the equation for the reaction is :
        H₂Z_(aq) + 2NaOH_(aq)  → Na₂Z_(aq) + 2H₂O_(l)
        Determine the number of moles of the dibasic acid H₂Z used 3mks
     4. Work out the concentration of solution X₂ in moles per dm³. 3mks
     5. Given that the concentration of solution X₂ is 8.17g/dm³ determine the relative formula mass of H₂Z. 3mks
     6. 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 .dived 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	15.0	14.9	15.0

   Complete the table ✓1½
   Consistency in the use of decimal places ✓1½
   Accuracy ✓2  (Titre should be around 15.0cm³)
   1. 15.0 + 14.9 = 14.95cm³ (correct principle of averaging)
              2
   2. No. of moles of NaOH =  25   × 0.1 ✓1 = 0.0025moles ✓2
                                        1000
   3. Mole ratio H₂Z: NaOH = 1:2 ✓1
      ∴ moles of H₂Z = 0.0025 ✓1 = 0.00125moles ✓1
                                   2
   4. 14.95cm³ H₂Z contains 0.00125moles
      ∴ 1000cm³ H₂Z contains 1000 × 0.00125 =0.084M
                                          14.95
   5. Molar Mass = concentration =  8.17   = 97
                            Molarity           0.084
   6. (1×2) + Z = 97
              2+Z = 97
                   Z= 95
2.  
   

   Test	observation	Inferences
   Observe solid M and describe its appearance	Blue crystals	M is likely to be a Copper (II) salt
   Take half of solid M in a clean dry boiling tube and heat it gently	White powder formed colourless liquid formed	M is hydrated salt
   Take the remaining solid M in to a boiling tube and add about 5cm3 of distilled water and shake .dived the solution formed in to two portions.	Solid dissolves forming pale-blue solution	M is likely to be a Copper (II) salt
   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.	Cu2+ Present
   To the second portion add a few drops of barium chloride followed by a few drops of dilute hydrochloric acid.	White precipitate formed insoluble in dil. HCl	Cu2+ Present

   Cations -Cu²⁺ ✓1½ 
   Anions - SO₄²⁻  ✓1½