Instructions to candidates
- Answer ALL the questions
- You are not allowed to start working with the apparatus for the first 15 minutes of the 2¼ hours allowed for this paper. This time is to enable you to read the question paper and make sure you have all the chemicals and apparatus that you may need.
- All working MUST be clearly shown where necessary.
- Mathematical tables and silent electronic calculators may be used.
QUESTION 1
You are provided with the following
- Metal P, Magnesium ribbon.
- Solution Q, Hydrochloric acid.
- Solution R, 0.50M sodium hydroxide,
Procedure I.
- Place five test tubes in a test tube rack and label them as 1, 2, 3, 4 and 5.
- Using a measuring cylinder, measure out volumes of hydrochloric acid as shown in the table and pour into the test tubes.
- Using a measuring cylinder add 1 cm3, 2 cm3, 3 cm3 and 4cm3 into test tube 2, 3, 4, and 5 respectively.
- Cut out five pieces of 1 cm long of metal P.
- Place one piece of magnesium into test-tube one and start a stopwatch immediately.
- Record the time taken for the metal to react completely in the table below.
- Do not pour the solutions in the test-tubes away for use in procedure II.
Table I-
Test tube 1 2 3 4 5 Volume of solution Q 10 9 8 7 6 Volume of water added 0 1 2 3 4 Time taken (sec) Rate of reaction 1/t (sec−1) - Plot a graph of rate of reaction against volume of solution Q used. (3 marks)
- From the graph, determine the time taken for 1 cm long magnesium ribbon to react completely if volume of acid used is 7.5 cm3. (1 mark)
- In terms of rate, explain the shape of the graph. (1 mark)
-
Procedure II
- Transfer all the contents in the five test-tubes into a volumetric flask. Rinse the test tubes and transfer the contents to the 250ml flask. Add water up to the 250ml mark. Label the solution T.
- Pipette 25cm3 of solution T. Add 2-3 drops of phenolphthalein indicator. Fill the burette with solution T. Titrate and record your results in the table below.
Table II
I II III Final burette reading (cm3) Initial burette reading (cm3) Volume of solution T used (cm3) - Calculate the average volume of solution R used. (1 mark)
- Calculate the number of moles of solution R used. (1 mark)
- Calculate the number of moles of solution T used. (1 mark)
- Calculate molarity of solution T. (1 mark)
- Calculate the mass of magnesium ribbon used in the experiment. (1 mark)
QUESTION 2
You are provided with solid D. Carry out the following tests on D and record your observations and inferences in the spaces provided.
-
- Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
Observations Inferences (½mark) (½mark) - To the first portion, add sodium hydroxide solution dropwise until in excess.
Observations Inferences (½mark) (½mark) - To the second, add Ammonia solution dropwise until in excess.
Observations Inferences (½mark) (½mark) - To the third portion add a few drops of Lead (II) nitrate solution.
Observations Inferences (½mark) (½mark) - To the fourth portion add drops of barium Nitrate solution followed by drops of nitrate acid solution.
Observations Inferences (½mark) (1 mark)
- Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
- Place the residue in a boiling tube and add dilute nitric (V) acid little by little until all the solid dissolve. Divide the solution into two parts.
Observations Inferences (½mark) (½mark) - To the first portion, add sodium hydroxide dropwise until in excess.
Observations Inferences (1 mark) (1 mark) - To the second portion add ammonia solution dropwise until in excess.
Observations Inferences (1 mark) (1 mark)
- To the first portion, add sodium hydroxide dropwise until in excess.
QUESTION 3
You are provided with solid Z. Carry out the tests below and record your observations and inferences in the spaces provided.
- Scoop a little of solid A with a clean metallic spatula and place it a non-luminous flame.
Observations Inferences (1 mark) (1 mark) - Add about 10cm3 of distilled water to the remaining solid Z in a boiling tube. Divide the resulting solution into 4 portions.
- To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
Observations Inferences (1 mark) (1 mark) - To the second portion, add 3 drops of bromine water and warm.
Observations Inferences (1 mark) (1 mark) - To the third portion, add 2 - 3 drops of universal indicator and determine the pH of the solution.
Observations Inferences (1 mark) (1 mark)
- To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
Marking Scheme
QUESTION 1
You are provided with the following
- Metal P, Magnesium ribbon.
- Solution Q, Hydrochloric acid.
- Solution R, 0.50M sodium hydroxide,
Procedure I.
- Place five test tubes in a test tube rack and label them as 1, 2, 3, 4 and 5.
- Using a measuring cylinder, measure out volumes of hydrochloric acid as shown in the table and pour into the test tubes.
- Using a measuring cylinder add 1 cm3, 2 cm3, 3 cm3 and 4cm3 into test tube 2, 3, 4, and 5 respectively.
- Cut out five pieces of 1 cm long of metal P.
- Place one piece of magnesium into test-tube one and start a stopwatch immediately.
- Record the time taken for the metal to react completely in the table below.
- Do not pour the solutions in the test-tubes away for use in procedure II.
Table I-
Test tube 1 2 3 4 5 Volume of solution Q 10 9 8 7 6 Volume of water added 0 1 2 3 4 Time taken (sec) 52.70 150.03 165.70 254.90 420.5 Rate of reaction 1/t (sec−1) 0.01923 0.006667 0.006061 0.003937 0.002381 - Plot a graph of rate of reaction against volume of solution Q used. (3 marks)
- From the graph, determine the time taken for 1 cm long magnesium ribbon to react completely if volume of acid used is 7.5 cm3. (1 mark)
1/t =0.56 × 10−2
t=178.57seconds - In terms of rate, explain the shape of the graph. (1 mark)
- An increase in the volume of acid leads to a consequent incraese in the rate of reaction (shape must be curved directly propotional to concentration.
-
Procedure II
- Transfer all the contents in the five test-tubes into a volumetric flask. Rinse the test tubes and transfer the contents to the 250ml flask. Add water up to the 250ml mark. Label the solution T.
- Pipette 25cm3 of solution T. Add 2-3 drops of phenolphthalein indicator. Fill the burette with solution T. Titrate and record your results in the table below.
Table II
I II III Final burette reading (cm3) 15.5 15.6 15.5 Initial burette reading (cm3) 0.0 0.0 0.0 Volume of solution T used (cm3) 15.5 15.6 15.5 - Calculate the average volume of solution R used. (1 mark)
15.5 +15.5 =15.5cm3
2 - Calculate the number of moles of solution R used. (1 mark)
0.5moles→1000cm3
? →15.5cm3
15.5 × 0.5
1000
=0.00775moles - Calculate the number of moles of solution T used. (1 mark)
MgCl2:NaOH
1:1
0.00775 =0.003875moles
2 - Calculate molarity of solution T. (1 mark)
0.003875 → 2.5cm3
? → 1000cm3
1000 × 0.003875
2.5
=0.155M - Calculate the mass of magnesium ribbon used in the experiment. (1 mark)
- Calculate the average volume of solution R used. (1 mark)
QUESTION 2
You are provided with solid D. Carry out the following tests on D and record your observations and inferences in the spaces provided.
-
- Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
Observations Inferences Colourless filtrate
Black (1mark)Fe2+,Cu2+,Fe3+ absent
Cu2+ (1mark) - To the first portion, add sodium hydroxide solution dropwise until in excess.
Observations Inferences White precipitate,soluble in excess (1mark) Zn2+,Al3+,Pb2+ (1mark) - To the second, add Ammonia solution dropwise until in excess.
Observations Inferences White precipitate,soluble in excess (1mark) Zn2+ (½mark) - To the third portion add a few drops of Lead (II) nitrate solution.
Observations Inferences White precipitate (½mark) SO42−, SO32−, Cl− ,Br− (1mark) - To the fourth portion add drops of barium Nitrate solution followed by drops of nitrate acid solution.
Observations Inferences White PPT Insoluble (1 mark) SO42− (½ mark)
- Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
- Place the residue in a boiling tube and add dilute nitric (V) acid little by little until all the solid dissolve. Divide the solution into two parts.
Observations Inferences Black and dissolves forming blue solution (½mark) Cu2+ (½mark) - To the first portion, add sodium hydroxide dropwise until in excess.
Observations Inferences Blue PPT insoluble in excess (1 mark) Cu2+ (½ mark) - To the second portion add ammonia solution dropwise until in excess.
Observations Inferences Blue PPT dissolves to form deep blue solution (1 mark) Cu2+ (½ mark)
- To the first portion, add sodium hydroxide dropwise until in excess.
QUESTION 3
You are provided with solid Z. Carry out the tests below and record your observations and inferences in the spaces provided.
- Scoop a little of solid A with a clean metallic spatula and place it a non-luminous flame.
Observations Inferences Burna yellow flame (1 mark)
Long chain hydrocarbon (1 mark) - Add about 10cm3 of distilled water to the remaining solid Z in a boiling tube. Divide the resulting solution into 4 portions.
- To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
Observations Inferences Orange persists (1 mark) R-OH absent (1 mark) - To the second portion, add 3 drops of bromine water and warm.
Observations Inferences yellow bromine
Yellow persist (1 mark)
Absent (1 mark) - To the third portion, add 2 - 3 drops of universal indicator and determine the pH of the solution.
Observations Inferences 1, 2, 3 (1 mark) Strongly acidic (1 mark)
- To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
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