Chemistry Paper 3 Questions and Answers - Form 3 End Term 3 2022 Exams

QUESTIONS

1. (24 Marks)
   • You are provided with:-
   • Aqueous hydrochloric acid solution P in a beaker.
   • A solution Q containing 6.3 g of dibasic acid H₂C₂O₄.2H₂O per litre.
   • Aqueous sodium hydroxide solution R.
   • You are required to:
     1. Standardize the sodium hydroxide solution R.
     2. Use the standardized solution R to determine the concentration of solution P in moles per litre.

PROCEDURE I
Fill the burette with solution Q. Pipette 25.0 cm³ of solution R into a conical flask and 3 drops of phenolphthalein indicator. Titrate this solution with solution Q until the pink colour just disappears .Record your results in table I below .Repeat this procedure two more times and complete table 1.
TABLE 1

(4 marks)

1. Calculate the average volume of solution Q used.   (1 mark)
2. Calculate the concentration of the dibasic acid solution Q in moles per litre. (2 marks)
   (C=12,O=16,H=1)
3. Calculate the number of moles of the dibasic acid solution Q used.(1 mark)
4. Calculate the number of moles of sodium hydroxide in 25cm³ of solution R.(2 marks)
5. Calculate the concentration of sodium hydroxide solution R in moles per litre (2 marks)

PROCEDURE II
Using a 100cm³measuring cylinder, measure 90cm³of distilled water and transfer it into a 250cm³ beaker. Dry the measuring cylinder and use it to measure 10cm³ of solution P and add it to the 90 cm³ of distilled in the beaker. Mix the solution well and label it solution T.
Fill the burette with solution T and pipette 25cm³ of solution R into a clean conical flask. Add 3 drops of phenolphthalein indicator. Titrate this solution with solution T until the pink colour just disappears .Record your results in table II below .Repeat this procedure two more times to complete table II.
Table II

1. Calculate the average volume of solution T used. (1 mark)
2. Calculate the number of moles of solution R used.(1 mark)
3. Calculate the number of moles of hydrochloric acid in solution T that reacted completely with 25cm³ of Sodium hydroxide solution R. (2 marks)
4. Calculate the number of moles of hydrochloric acid in 100cm³ of solution T(2 marks)
5. Determine the concentration of the original hydrochloric acid solution P in moles per litre.(2 marks)

2. (16 MARKS)
You are provided with solid A, which is a mixture of two compounds .Carry out the experiment s below .Record your observations and inferences in the table and identify any gas (es) evolved.

(a)Transfer a spatula end-full of solid A in a boiling tube and add 20cm³of distilled water. Shake thoroughly and filter .Rinse the residue with distilled water and keep both the filtrate and the residue.

1 mark             1mark
(b)  Divide the filtrate into three portions, each of 2cm³.
(i)   To portion one, add ammonia solution dropwise until in excess.

1 mark             1mark
(ii)  To portion two, add 4 drops of 0.5M Lead (II) nitrate solution.

1 mark             1mark
(iii)To portion three, add 4 drops of Barium chloride solutions, followed by 5 cm³ of nitric (v) acid.

1 mark             1mark
c) Scrap the residue from the filter paper (a) above using a spatula and transfer it into a boiling tube. Add  to it  1M Nitric acid until it dissolves .Keep the resulting solution for use in part(d) below.

1 mark             1mark
(d)  Divide the solution into 3 parts.
(i)   To part two, add ammonia solution dropwise.

1 mark             1mark
(ii) To part two, add 4 drops of 1M hydrochloric acid solution.

1 mark             1mark
(iii) To part three, add 4 drops of potassium Iodide solution.

1 mark             1mark

MARKING SCHEME

PROCEDURE I
TABLE I ……………. 5 Mks distributed as follows.

1.                  
   • Complete table (1 Mark)
     1. Complete table with 3 titrations (1 Mark)
     2. Incomplete table with 2 titrations ( ½ Mark)
     3. Incomplete table with 1 titration (0 Mark)
   • Penalties
     1. Wrong arithmetic
     2. Inverted table
     3. Burette reading beyond 50cm³ unless where explained.
     4. Unrealistic titre values below 1cm³ or above 50cm³.
        NB: Penalise ½ mark each to a maximum of ½ mark.
2. Use of decimals (tied to 1st and 2nd rows only) (1 Mark)
   1. Accept 1 or 2 d.p used consistently otherwise penalise fully.
   2. If 2 d.p is used, the 2nd d.p must be either ‘0’ or ‘5’ otherwise penalise fully.
3. Accuracy (Tied to correct titre value) (1 Mark)
   Compare the candidate titre values with the teacher titre values and mark as follows;
   1. If at least one is within ± 0.1 of the teacher’s value. (1 Mark)
   2. If none is within ± 0.1, but atleast one is within ± 0.2 of teacher’s value ( ½ Mark)
      NB: If there are 2 possible values from the teacher’s result, use one that is closer to candidate’s value
      and mark accordingly.
4. Principles of averaging (1 Mark)
   Conditions
   1. If 3 consistent values are averaged (1 Mark)
   2. If 3 titrations are done but only two are consistent and averaged (1 Mark)
   3. If 2 titrations done and are consistent and averaged. (1 Mark)
   4. If 3 titrations are done but are inconsistent and averaged. (0 Mark)
      NB: Values averaged must differ by a maximum of ± 0.2cm³ of each other.
5. Final answer (1 Mark)
   (Tied to correct average titre)
   Compare the candidate’s average titre with the teacher’s average titre.
   1. If within ± 0.1 of teacher’s value. (1 Mark)
      (ii) If not within ± 0.1 but within ± 0.2 of the teacher’s value ( ½ Mark)
      (iii) If none is within ± 0.2 (0 Mark)
      NB:
      Accept final answer expressed to 2 d.p unless the answer works out to an exact figure otherwise penalise ½ Mark

PROCEDURE II

1. Table II – 5 Marks
   NB: Marks as per table I
   Calculations
   PROCEDURE I
2.  Rmm of H2C2O4.2H2O = 126
   . :Molarity of solution Q in moles per litre = 6.3/126 1 = 0.05 1
3. Moles of dibasic acid solution Q used = (Average volume x 0.05 (ans i)) ½
                                                                                      1000
   Correct answer ½
4. 2NaOH(aq) + H₂C₂O₄(aq) → Na₂C₂O₄(aq) + 2H₂O(l)
   Acid : Base = 1:2 1
   . :Moles of sodium hydroxide used = 2 x ans iii ½
   Correct answer ½
   NB:
   If the equation is missing award zero marks for the working.
5. Concentration of sodium hydroxide solution R = (1000 x Ans iv)
                                                                                          25
   Correct ans. 1
   NB: Ignore if units missing but penalise ½ mark for wrong units used.

PROCEDURE II

1.                            
2. Moles of solution R used = (25 x Ans (v) )  ½ = Correct ans ½
                                                  1000
3. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) 1
   Acid : Base = 1:1 1
   . : Moles of HCl in solution T that reacted with 25cm3 of solution R = Ans ii.
   NB: If Ans ii) is transferred intact to iii) but the equation is missing award zero marks.
4. Moles of HCl in 100cm3 of solution T =                   (100 x Ans iii)                  1 = Corr. Ans 1
                                                                      (Average volume of solution T)
5. Original concentration of HCl in solution P = (1000 × Ans iv) = Corr. Ans 1
                                                                                 10
   NB: If wrong units are used penalise ½ mark but ignore if units are missing.

2. 16 Marks