QUESTION 1
You are provided with
- Aqueous sulphuric (VI) acid ,solution X
- 4.0g of sodium carbonate to make 500cm3 of solution , solution Y
- An aqueous solution of substance M, solution M
You are required to determine the;
- Concentration of solution X
- Enthalpy of reaction between sulphuric (VI)acid and substance M
Procedure I
Using a 50ml measuring cylinder, measure 25.0cm3 of the solution X and transfer into 250 ml volumetric flask. Add water to the mark to make 250cm3 of solution. Label this as solution L. Fill a burette with solution L . Pipette 25.0cm3 of solution Y into a conical flask. Add 3 drops of methyl orange indicator provided and titrate with solution L until a permanent yellow colour just appears. Record your result in table I below. Repeat the titration two more times and complete the table I below.
Table 1
| I | II | III | |
| Final burette reading (cm3) | |||
| Initial burette reading (cm3) | |||
| Volume of solution L used (cm3) |
Calculate the ;
- Average volume of solution L used (1mk)
- Concentration of sodium carbonate, solution Y in moles per litre (Na=23,C=12,O=16) (1mrk)
- the number of moles of sodium carbonate that reacted. (1mk)
- Concentration of sulphuric (VI) acid in solution L in moles per litre. (2mks)
- Concentration of Sulphuric (VI) acid in solution X in moles per litre. (1mk)
PROCEDURE B
Label six test tubes as 1, 2,3,4,5, and 6. Using a measuring cylinder, measure 2cm3 of solution X into test –tube number 1, 4cm3 in test –tube number 2. Continue with this process for all the other test tubes as shown in table II below.
Clean the burette and fill it with solution M. From the burette, Place 14cm3 of solution M into a boiling tube .Measure the initial temperature of this solution and record it in the table II below to the nearest 0.5°C. Add the content of test tube number 1 to the boiling tube containing solution M . Stir the mixture with thermometer and record the highest temperature reached in table II below .Repeat the process with the other sample of solution M given in the table II and complete the table.
Table II
| Test tube number | 1 | 2 | 3 | 4 | 5 | 6 |
| Volume of solution X(cm3) | 2 | 4 | 6 | 8 | 10 | 12 |
| Volume of solution M (cm3) | 14 | 12 | 10 | 8 | 6 | 4 |
| Highest temperature of mixture (°C) | ||||||
| Initial temperature of solution M (°C) | ||||||
| Change in temperature ,ΔT (°C) |
- On the grid below, draw a graph of ΔT (vertical axis ) against volume of solution X. (3mks)
From the graph, determine;- the maximum change in temperature (1mk)
- The volume of X required to give the maximum change in temperature (1mk)
- Calculate the
- Number of moles of sulphuric (VI) required to give the maximum temperature change. (1mk)
- Molar enthalpy of reaction between sulphuric (VI) acid and substance M in kilojoules per mole of sulphuric (VI) acid. (Specific heart capacity =4.2J/g/K, density of solution =1.0g/cm3) (2mks)
QUESTION 2
You are provided with solid T. Carry out the tests below and write your observation and inferences in the spaces provided.
- Place the solid in a boiling tube and add about 10cm3 of distilled water and shake well.
Divide the mixture into 4 portions- To the 1st portion, add 5 drops of sodium hydroxide solution
Observations (1mk) Inferences (1mk) - Dip a glass rod in the solution and burn it over a non-luminous flame.
Observations (½mk) Inferences (½mk) - To the 2nd portion, add 5 drops of barium nitrate provided .(Retain for use in (iv)
Observations (1mk) Inferences (1mk) - To the mixture in (iii) above, add about 2cm3 of dilute nitric (V) acid provided.
Observations (1mk) Inferences (1mk) - To the 3rd portion, add 5 drops of acidified potassium dichromate (VI)
Observations (1mk) Inferences (1mk)
- To the 1st portion, add 5 drops of sodium hydroxide solution
QUESTION 3
You are provided with solid G .Carry out the tests below and record your observations and inference In the spaces provided.
- Using a metallic spatula, burn 1/3 of the solid G on a non-luminous Flame.
Observations (1mk) Inferences (1mk) - Place the remaining solid in a boiling tube. Add about 10cm3 of distilled water and shake until the solid dissolves .Divide the mixture obtained into 4 portions ,
- To the 1st portion, add solid sodium hydrogen carbonate provided.
Observations (½mk) Inferences (½mk) - Test the pH of the 2nd portion.
Observations (½mk) Inferences (½mk) - To the 3rd portion, add 3 drops of acidified potassium manganate (VII)
Observations (1mk) Inferences (1mk) - To the 4th portion ,add 4 drops of acidified potassium dichromate (VI)
Observations (1mk) Inferences (1mk)
- To the 1st portion, add solid sodium hydrogen carbonate provided.
CONFIDENTIAL
In addition to the apparatus and fittings found in the laboratory, each student will require the following:
- About 80cm3 of solution X
- About 100cm3 of solution Y
- About 70cm3 of solution M
- 1 pipette
- 1 burette
- 2 conical flask (250ml)
- A 250ml volumetric flask
- 1 thermometer (-10°C to 110°C)
- 8 test tubes and test tube rack
- 2 boiling tubes
- 10ml measuring cylinder
- 7 labels
- a test-tube holder
- Solid T (about 0.3g)
- Solid G (about 0.3g)
- Glass rod
- Metallic spatula
- Solid sodium hydrogen carbonate (about 0.2g)
- 500 ml distilled water
- 50ml measuring cylinder
Access To:
- Bunsen burner
- methyl orange indicator supplied with a dropper
- 2M sodium hydroxide supplied with a dropper
- Aqueous Barium nitrate supplied with a dropper
- 2M Nitric (v) acid supplied with a dropper
- universal indicator supplied with a dropper
- pH scale chart.
- Acidified potassium manganate (VII) supplied with a dropper.
- Acidified potassium dichromate (VI) supplied with a dropper.
NB:
- Solution X is prepared by dissolving 55ml of concentrated sulphuric (VI) acid in one litre of solution.
- Solution Y is prepared by dissolving 8g of anhydrous sodium carbonate in one litre of solution
- Sodium M is prepared by dissolving 80g of sodium hydroxide in one litre of solution.
- Acidified potassium manganate (VII) is prepared by dissolving 3.16g of KMnO4 in 600cm3 of 2M H2SO4 and made to one litre solution.
- Acidified potassium dichromate (VI) is prepare by dissolving 6g of K2Cr2O7 in 600cm3 of 2M H2SO4 and made to one litre solution.
- 2M bench reagent of Sodium hydroxide is prepared by dissolving 80g of sodium hydroxide in one litre of solution.
- Nitric (V) acid (2M) is prepared by dissolving 126ml in one litre of solution.
- Barium nitrate solution is prepared by dissolving 0.05g in one litre of solution.
- Solid T = sodium sulphite
- Solid G = Maleic acid.
MARKING SCHEME
QUESTION 1
TABLE 1
Complete table ------------------1mk
- Complete the table with 3 titrations done –(1mk)
- Incomplete table with 2 titrations done ---(1/2mk)
- Incomplete table with 1 titration done –(0mk)
Penalties
Wrong table
- Inverted table
- Unrealistic value
Penalize ½ mark for each to a maximum of 1/2mk
Decimals ---------------1mk
(Tied to the first and second row only )
Conditions
Accept either 1or 2 decimals points used consistently
If the 2nd decimal point is used can only be o or 5
Accuracy -------1mk
Compare any titre value in the 3rd row with the school value (sv)
Conditions
If within ±0.1cm3 of sv ---1mk
- If not within ±0.1cm3 but within +0.2cm3 of sv ---½ mk
- Beyond ±0.2cm3 of sv -----omk
NB/ if there is wrong arithmetic in the table compare the sv with the correct value and credit accordingly.
Principle of averaging ----1mk
Value average must be shown and must be within ±0.2 cm3 of each other conditions.
3 values averaged and consistent -1mk
- 3 values done and only 2 possible averaged –1mk
- 2 titrations done and averaged -1mk
- 2 titrations done and inconsistent -0mk
- 3 titrations done and consistent but only two averaged -0mr
Final accuracy -1mk
Compare with the (sv )
If within +_0.1 of sv-1mk
- If within +_0.2 of sv -1/2mk
- If beyond +_0.2 of sv -0mk
NB// If the candidate has averaged wrong values pick the correct value if any ,average and credit accordingly
(ii) 106g of Na2CO3 =1 mole
8 = 8x1 ½
106
106
=0.0755M Na2CO3 ½
iii)In 1000cm3 of Na2CO3 = answer in (ii)
in 25cm3 = 25 x answer in (ii) ½
1000
= answer in (iii) ½
iv) Na2CO3(aq) + Na2CO3(aq)) → Na2SO4(aq) + CO2(g) + H2O(l)
Ratio Na2CO3.H2SO4
1 : 1
answer in (iii) : answer in (iii) ½
average titre in cm3 = answer in (iii)
In 1000cm3 = 1000x answer in (iii) ½
Average titre
= answer in (iv) M ½
v) Concentration of sulphuric (vi) acid
in 1000cm3 ………………………… answer in (iv)
in 1000cm3 ………………………… answer in (iv)
250cm3 ………………………………..?
answer in (iv) ×250 = answer in moles
1000
in 25cm3 = answer in moles
in 1000cm3 = ?
answer in moles x1000 = nswer in (v) M ½
25
Alternatively
M1V2 = M2V2
answer in (iv) X 250 = M2 X 25
M2 = answer in (iv) X 250
25
= answer in (V)
Conditions
If units given they should be correct however if not given ignore.
- Molarity should be given to at least 3 decimals place otherwise penalize ½ marks for the answer.
- Numbers of moles should be given to at least 4 decimal places, otherwise penalize ½ mark for answer.
TABLE II
- Complete table -2mrks
5-6 experiments done -2mks
3-4 experiments done -1 mark
Less than 3 experiments done -0mrk - Penalties
Penalize ½ mark for inverted table
Penalize fully for unrealistic temperature readings i.e. Above 50°c or below 10°c . - Decimals -1mark
Must be 1 dp or whole numbers used uniformly.
The first digit after the decimal must be a zero or 5 otherwise penalize fully.
Award either 1mk or zero. No half mark. - Accuracy -1mark
Accuracy is pegged on the candidate initial temperature reading
Conditions Award 1 mark if the candidate value is within ±2units the school value.initial temperature reading should be the same for all the six experiments otherwise penalize fully. - Trend -1mrk (tied to ΔT)
Award1mark for a continuous rise followed by continuous drop.
Award 1mark for a continuous rise, a constant then followed by continuous drop
GRAPH-3mks
- Labeling – ½ mrk
The vertical and horizontal axis must be correctly labeled with correct unit otherwise penalize fully - Scale – ½ mark
The actual plot must cover at least eight big squares on the vertical axis and at least 8 ½ big squares on the horizontal axis, otherwise penalize fully. - Plotting
5-6 correct plotted points -1mark
4 correctly plotted points- ½ mark
Less than 4 points plotted – 0mark - Line -1mrk
A straight line showing a continuous rise followed by a line showing a continuous drop.
Condition the two lines must be extrapolated above the last point.
-
- The Δt must be read from a correctly drawn graph.
- ΔT is correctly shown on the graph but not read correctly, award accordingly ½ mk.
The graph must be extrapolated above the last point.
- The volume of X must be read from a correctly drawn graph .
The reading must be shown on the graph. Without showing, penalize fully.
(iii)
I Answer in (ii) above x answer (v) in procedure I ½
I Answer in (ii) above x answer (v) in procedure I ½
1000
=correct answer 1 ½
II a) Δ H = mcΔT
= 16 x 4.2xΔT (answer (ii)I above )1 ½
1000
=correct answer ½
b) = 1 x correct answer above ½
Answer (iii)I above
= correct answer ½ in kJmol-1
Penalize ½ mk if wrong/no units or lack of –ve signs. Reject KJ
Penalize ½ mk if wrong/no units or lack of –ve signs. Reject KJ
QUESTION 2-SOLID T
| OBSERVATIONS | INFERENCES |
| (i) No white precipitate 1mk | Zn2+, Al3+, Pb2+ , Ca2+, Mg2+, Ba2+ absent, award 1 mk |
| (ii) burns with a yellow flame 1/2mk | Na+ present 1/2mk |
| (iii) white precipitate 1mk | CO32−, SO32−, SO42− present
3 ions 1mk
2 ions ½ mk
1 ion 0mk
Penalize ½ mk for any contradictory ion to a maximum of 1mk
|
| (iv) white precipitate dissolves / effervescence/ bubbles of a colourless gas. 1mk |
CO32−, SO32− present
2 ions - 1mk
1 ion – 1/2 mk
SO42− absent, award ½ mk.
Penalize ½ mk for any contradictory ion to a max of 1mk
|
| (v) orange colour of acidified K2Cr2O7 changes to green 1mk
Candidate must mention original and final colour and the name of solution, otherwise penalize fully.
|
SO32− present 1mk (must be inferred in iv above) otrherwise award 0mk
Penalize fully for any contradictory ion
|
QUESTION 3-Solid G
| OBSERVATIONS | INFERENCES |
| a) Burns with a yellow luminous /sooty/ smoky flame 1mark | ̦̀C=C̦΄ or -C=C- present 1mk or
- unsaturated organic compound 1mk or
hydrocarbon with high C:H ratio 1mk or
-aromatic organic cpd
|
| b) Effervescence /bubbles/ fizzing ½
reject: hissing/fizzling/sizzling
|
R –COOH / H+ / H3O+ present ½
Penalize fully for any contradictory ion
|
| ii) pH = 4or 5 | Weakly acidic |
| iii)purple KMnO4 turns colourless | ̦̀C=C̦΄ or -C=C- ½ mk present , ROH 1/2 mk |
| (iv) Orange colour of acidified K2Cr2O7 remained 1mk |
̦̀C=C̦΄ or -C=C- 1 mk
Accept for ½ mk ROH absent
|
̀
NB//
- Penalize fully for- C=C- and C= or H-C=C-H and H-C=C-H
- The pH value should not be a range of values e.g. 4-5
- Penalize fully for weak acid in the inference of b(ii)
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