CHEMISTRY PAPER 3 - 2019 LANJET JOINT MOCK EXAMINATION

Q1. You are provided with:

• Solution A: dibasic acid H₂C₂O_4●XH₂0 containing 2.52g in 250cm³ of solution.
• Solution B: 0.2MNaOH.

You are required to determine the value of X in the formula H₂C₂O_4●XH₂0   (H=1, C=12, O=16)
Procedure
Fill the burette to the mark with solution A, pipette 25cm³ of solution B into a clean dry conical flask. Titrate solution A against solution B using phenolptherine as indicator. Repeat the titration to obtain consistent results and fill table below.

1. Calculate the average volume of solution A.   (5mks)
2. Calculate the Molarity of the solution A. (2mks)
3. Calculate the relative formula mass of the acid solution A and hence the volume X in H₂C₂O₄XH₂O.     (3mks)

Q2. You are provided with;

• Solution C: 0.2M potassium iodide solution
• Solution D: 0.2M sodium thiosulphate solution
• Solution E: Hydrogen peroxide
• Starch indicator solution.

You are required to determine the effect of concentration on rate of a reaction.
Procedure
Transfer 10cm³ of potassium iodide into test tube labeled C using a burette 5xm³ of sodium thiosulphate into test tube labeled D and 2cm of hydrogen peroxide into test tube labeled E. Using 10ml measuring cylinder and clean it before using it to measure next solution.
Transfer solution B into clean conical flask followed by 5cm³ of fleshly prepared starch solution. Simultaneously add solution C and solution E to the conical flask and immediately start to turn blue black. Record your results in the table that follows. Repeat the process using different volumes of solution E and complete table below.

1. Plot the graph of ¹/_t sec ^-1 versus volume of H₂O₂ (6mks)
2. From the graph determine the time taken for the black to appear if volume of hydrogen peroxide solution E and cm is 4.5cm³.   (3mks)
3. Explain the effect of concentration to rate of reaction.   (1mks)

Q3. you are provided with solid F. Carry out the following tests and record the observations and inferences in the space provided.

1. Place about one third solid F into clean dry test tube and heat it strongly.
   

   Observation	Inference
   (1mk)	(1mk)

2. Place the remaining solid F in a boiling tube. Add about 10cm³ of distilled water. Shake the mixture thoroughly for about one minute. Filler and divide the filtrate into four portions.
   

   Observation	Inference
   (1mk)	(1mk)

3. To the first portion add two drops of phenolphthalein indicator.
   

   Observation	Inference
   (1mk)	(1mk)

4. To the second portion, add two drops of hydrochloric acid.
   

   Observation	Inference
   (1mk)	(1mk)

5. To the third portion, add about three drops of sodium sulphate solution.
   

   Observation	Inference
   (1mk)	(1mk)

6. To the last portion, add drops of sodium hydroxide drop wise until is excess.

You are provided with solid G. Carry out the following tests and record observation and inferences in space provided.

1. Place about one third of sodium G into clean metallic spatula and burn it in a Bunsen burner flame.
   

   Observation	Inference
   (1mk)	(1mk)

2. Place the remaining Place the remaining of solid G in a boiling tube. Add about 10cm³ of distilled water and shake. Divide the mixture four portions.
   

   Observation	Inference
   (1mk)	(1mk)

3. To the first portion of mixture in test tube and solution NaHCO₃ provided.
   

   Observation	Inference
   (1mk)	(1mk)

4. To the second portion add three drops of acidified potassium manganate VII.
   

   Observation	Inference
   (1mk)	(1mk)

5. To the last portion add three drops of bromine water.
   

   Observation	Inference
   (1mk)	(1mk)

MARKING SCHEME

1. Complete table CTü
   Condition – 3 titration doneü
   Incomplete table with 2 titration ü½
   Incomplete table with 1 titration. 0mk
   Decimal Dü
   Conditions – Candidate value to be compared with school value.
   • Value with ± 001 of s.vü
   • Value with ±2 of s.v ü½
   • Value beyond ±2 of school value. 0mk
     
     
   • Accuracy A ü - 30.8cm³
   • Compare with school value. + or – 0.1ü+ or – 0.2 ü½
     Principles of averaging P.Aü
     Conditions - Value average must be shown at D within 0.2cm³ of each other.
   • If 3 titrations done and are within ± 0.2 but only 2 averaged, award 0mk
     Find answer F.A ü½
     The find answer after averaging must be shown with or without the units.
     1. Moles of NaOH ≡> 24 x 0.2 = 0.005molesü½
                                      1000
        Moles of acid (moles ratio 1:2) = 0.005/₂ = 0.0025molesü½
        Molarity of the acid ≡> 0.0025 X 1000ü½
                                        30.8 (titre value)
                                  => 0.081mm/Lü½
     2. 2.52g = 250cm³
             ?     = 1000cm³
        2.52 x 1000 = 10.08g/lü
           250
        Molarity 1- mol/l = g/l
                                    RFM
        RFM = 10.08/_0.081 = 124.4
        H₂C₂O4 X H₂O = 124.4ü
        1 x 2 + 12 x 2 + 16 x 4 + x (1 x2 +16) = 124.418x = 34.4
        X= 1.9 that is 2ü
2.  
   

   Set	Vol of C	Vol of D	Vol of E	Vol of starch	Time (sec)	1/t sec -1
   1			2		160	0.006
   2			3		94	0.010
   3			4		82	0.012
   4			5		71	0.014
   5			6		60	0.017

   CT (complete table) ü (4mks)
   D (decimal atleast 3dp for 1/t) ü
   T (Trend) ü = (1mks)
   Penaties – any unfilled space _ ½
                -Values in fractions for 1/t penalize total of 1mk
   1. 1/t = 0.0125 (must be shown on graph paper)
         t = 80sec ü½
   2. When concentration is high the rate of reaction is high since there are more of the reaction particles. ü
3.  
   1.  
      1. - White crystal turns to white powder ü½
         -Colouless vapour condenses on the cooler part ü½   - Hydrated compound saltü
      2. Colourless fitrates
         White residue             ü½           - Solid sparingly soluble ü½ or Solid mixture of soluble as insoluble salt
      3. Phenolphthalein turns pinkü     - OH^-SO^2-₃ CO₃^2-, HCO^-₃
         NB All 4 ions ü
         2 ions ü½
                    1 ion 0
      4. No effervescence // no bubbling         
         CO₃^2- HCO^-₃ Absentü½ (if OH^- present ü)
         OH^- Present ü (if CO₃^2-, HCO^-₃ present ü½)
      5. White precipitate ü½ - Ca²⁺, Ba²⁺ presentü½
         if pb²⁺ Mentioned or Ag⁺ ignore
      6. White precipitateü½ Ca²⁺ or Ba²⁺ Present ü
         Insoluble in excess                                                   
   2.  
      1. solid melts and burns with sooty /smocky flame. ü C = C or C ≡ Cü
      2. Solid dissolved forming clourless solutionü - polar organic compound ü
         Polar compound.
         Effervescenceü½// bubbling                               
         - R-COOH presentü½ (ignore H⁺ or H₃O⁺present)
   3. Purple potassium manganate vii turns colourless               - C =C or- C≡ C-
      R - OHü½
      Orange bromine water turns colorless. ü – C = C or C≡ Cü present
      Accept yellow bromine turns colourless