Questions
Instructions to candidates
- Answer ALL questions in the spaces provided in the question paper
- KNEC Mathematical tables and silent non – programmable electronic calculators may be used.
- All working MUST be clearly shown where necessary.
- Candidates should answer the questions in English.
QUESTIONS 1
You are provided with:
- Solution C1 which is a solution of a dibasic acid, H2C2O4.XH2O containing 5.04g in 500cm3 of solution.
- Solution C2 which is a 0.2M solution of sodium hydroxide.
You are required to determine the value of X in the formula H2C2O4.XH2O. (H=1.0;C=12.0;O=16.0). - Procedure:
Fill the burette to the mark with solution C1. Pipette 25.0cm3 of C2 into a clean dry conical flask.Titrate C1 against C2 using phenolphthalein as indicator. Repeat the titration to obtain consistent results. Enter your results in the table below. (4mks)
Titration Number 1 2 3 Final burette reading (cm3) Initial burette reading (cm3) Volume of acid used (cm3) - Calculate the average volume C1 used. (1mark)
- Calculate the concentration of the acid in moles per litre. (3marks)
- Calculate the relative formula mass of the acid. (2marks)
- Hence determine the value of X in the acid H2C2O4.XH2O. (2marks)
QUESTIONS 2
You are provided with:
- 2cm Magnesium ribbon Solid A.
- About 60cm3 Solution B (0.7M hydrochloric acid)
You are required to determine the:- Temperature change when magnesium reacts with excess hydrochloric acid.
- Molar heat of reaction between Magnesium and hydrochloric acid.
- Procedure:
Using a burette, measure 50cm3 of Solution B and place in a 100ml beaker. Measure the temperature of Solution B in the 100ml beaker and record the value in table II below. Put the magnesium ribbon in the 50cm3 of Solution B in the 100ml beaker and immediately start the stop watch. Stir the mixture continuously with the thermometer making sure the magnesium ribbon remains inside the solution as it reacts. Measure the temperature after every 30seconds and record in the values in the table below. Continue stirring and measuring the temperature to complete the table.- Table II ( 5mks)
Time (Sec) 0 30 60 90 120 150 180 210 240 270 300 Temperature (°C) -
- Plot a graph of temperature (Y-Axis) against time. (3marks)
- On the graph, show the maximum change in temperature, ∆T and determine its value. (1mark)
- Determine the molar heat of reaction between magnesium and hydrochloric acid. (Assume the heat capacity of the solution is 4.2j/g/k and density is 1.0g/cm3. (3marks)
- Table II ( 5mks)
QUESTIONS 3
You are provided with solid E. Carry out the following tests on E and record your observations and inferences in the tables provided.
- Put solid E in a boiling tube. Add distilled water to half the tube. Shake the mixture.
Observations (1 mark) Inference (1 mark) -
- To about 1cm3 of solution formed, add sodium hydroxide drop wise until in excess.
Observations (1 mark) Inference (1 mark) - To about 1cm3 of the solution above, add ammonia solution drop wise until in excess.
Observations (1 mark) Inference (1 mark) - To about 1cm3 of the solution above, add universal indicator.
Observations (½ mark) Inference (½ mark) - To about 1cm3 of the solution above, add 3 drops of dilute nitric (iv) acid.
Observations (1 mark) Inference (1 mark) - To about 1cm3 of the solution above, add Lead (II) nitrate solution and boil the mixture
Observations (1 mark) Inference (1 mark) - To about 1cm3 of the solution, add barium chloride solution.
Observations (1 mark) Inference (1 mark)
- To about 1cm3 of solution formed, add sodium hydroxide drop wise until in excess.
QUESTIONS 4
You are provided with solid F. Carry out the tests below and record your observations and inferences in the spaces provided.
- Place half of Solid F in a boiling tube. Add about 8cm3 of absolute ethanol and shake.
Observations (1 mark) Inference (1 mark) - Place about 2cm3 of obtained in a test tube. Add universal indicator solution and test its PH.
Observations (½ mark) Inference (½ mark) - Place about 2cm3 of obtained in a test tube. Add Acidified Potassium dichromate (vi).
Observations (1 mark) Inference (1 mark) - Place about 2cm3 of obtained in a test tube and add half of the Sodium hydrogen carbonate provided.
Observations ( ½ mark) Inference (½ mark) - Place the other half of Solid F in a boiling tube. Add 8cm3 of distilled water and shake.
- Place about 2cm3 of obtained in a test tube. Add Sodium Hydrogen Carbonate that remained.
Observations ( ½ mark) Inference ( ½ mark) - Place about 2cm3 of obtained in a test tube. Add Universal Indicator solution and test its PH.
Observations ( ½ mark) Inference ( ½ mark)
- Place about 2cm3 of obtained in a test tube. Add Sodium Hydrogen Carbonate that remained.
Confidential
Each candidate will require.
- About 100cm3 solutionC1 (H2C2O4.2H2O containing 5.04g in 500cm3 solution)
- About 100cm3 solution C2 (0.2M NaOH)
- About 2g solid E (ZnSO4)
- 2 cm long solid A (Magnesium ribbon)
- About 60cm3 solution B (0.7M HCl)
- About 1 g solid F (maleic acid)
- About 8 cm3 absolute ethanol
- Burette
- Pipette
- Pipette filler
- 2 boiling tubes
- 5 test tubes
- Universal indicator with a chart
- 0.2g sodium hydrogen carbonate
- Distilled water
- 100ml glass beaker
- Thermometer( -100c to 1100c)
- Stop watch
- Means of labelling
- Spatula
Access to the following
- Phenolphthalein indicator
- Pottassium dichromate(vi)
- 2M NaOH
- 2M Ammonia solution
- 2M Nitric(v) acid
- Lead nitrate solution
- Barium chloride solution
- Bunsen burner
Marking Scheme
-
-
- Complete table (1mk)
- Conditions
- Complete table with 3 titration = 1 mk
- Incomplete table with 2 titration = ½mk
- Incomplete table with 1 titration = 0mk
- Penalties
- Wrong arithmetic/ subtraction
- Inverted tables
- Burette readings beyond 50.0cm3 unless explained
- Unrealistic titre values ie too low (less than 1cm3) or too high (more than 50.0cm3)
- Penalize ½ mk EACH to a maximum penalty of ½mk (penalize once ½ mk )
- Conditions
- Use of decimals (tied to rows 1 and 2 only) = 1mk
- Conditions
- Accept only 1 to 2dp used consistently, otherwise penalize fully.
- Accept 2dp only if 2nd dp is '0' or '5' otherwise penalize fully
- Accept inconsistency in use of zeros as initial volume i.e, 0, 0.0, 0.00
- Conditions
- Accuracy
- Only thick the correct value otherwise don't tick compare the candidates / students value with the school value (sv) and tick (✓) the chosen value where it earns a mark.
- Conditions
- If any value is within ± 0.10cm3 of s.v = (1mk)
- If no value is within ± 0.10cm3 but at least is within ± 0.20cm3 of s.v ... (½mk)
- If no value is within ± 0.20cm3 of s.v (0 mk)
If there is wrong arithmetic // subtraction in the table , compare the s.v with the worked out correct value// the titre and award accordingly.
- Principles of averaging
- Values averaged must be shown and must be with ± 0.20cm3 of each other.
- Conditions
- If three consecutive values are averaged .. (1 mk)
- If three titrations are done but only 2 are within 0.20cm3 of each other.... ( ½ mk)
- If only 2 titrations are consistent and averaged ( ½ mk)
- If 3 possible but only 2 are averaged (0mk)
- If 3 titrations are done, are consistent and yet all are averaged (0mk)
- If only 2 titrations are done , are inconsistent and yet averaged (0 mk)
- Penalties
- Penalize ½mk for wrong arithmetic error outside ±2 units in the second decimal place
- Penalize ½mk if no working is shown but the answer is correct.
- If no working is shown and the answer is wrong penalize FULLY.
- Accept rounding off the naswer to 2 decimal places, otherwise penalize ½mk for wrong rounding off to 1 dp or to whole numbers
- Final accuracy
- Compare candidates correct average titre as reflected by his table with the s.v and award accordingly.
- Conditions
- If the correct average is within ± 0.10cm3 of s.v ... award (1mk)
- If the correct average is not within ± 0.10cm3 of s.v but within ± 0.20cm3 of s.v ..award (½ mk)
- If the correct average is outisde of ± 0.20cm3 of s.v..award (0 mk)
- Penalties
- Penalize ½mk for rounding off to 2d.p or to whole number
- Penalize fully for wrong rounding off to 1 dp or to whole numbers.
- Table 1
- Complete table ...... 1mk
- Use of decimals ...... 1mk
- Accuracy .....1mk
- Principles of averaging.... 1mk
- Final accuracy.....1mk
- Complete table (1mk)
- Calculations
Concentration of acid
if 1000cm3 ≡ 0.2 moles
25 cm3 ≡ 2.5 x 0.2 = 0.05 moles
1000
Acid being dibasic mole ratio of acid: base = 1:2
Thus 2 moles NaOH reacts with 1 mole of acid
Then 0.005 moles reacts with 0.005 x 1 = 0.0025 moles
2
Average titre value ≡ 0.0025 moles
Then 1000cm3 ≡ 1000 x 0.0025
average titre value
ie 1000 x 0.0025
s.v
(otherwise penalize wrong units) - RFM of acid
- Molarity = mass/ litre (g/litre) Tus RMM = g/litre
RMM molarity
Mass per litre
500cm3 = 5.04 g → 1000 x 5.04 = 10.08 g
1000cm3 500
Thus RFM (RMM) = 10.08 (no units)
Ans (b) - Conditions
- Penalize ½mk for wrong transfer of moles and mass; otherwise penalize fully.
- Accept rounding off answer to at least 3 d.p otherwise penalize ½ mk
- Penalize ½mk for wrong transfer if the arithmetic error is outside ± 2 units in the 3d.p
- Penalize ½mk if units/ wrong units given
- Molarity = mass/ litre (g/litre) Tus RMM = g/litre
- Value of X
- H2C2O4 x H2O RMM = Ans in C above
2(1) + 2(12) + 4(16) ∴ 90 + 18x = Ans C above
2 + 24 + 64 = 90
18x = Ans C - 90
x = Ans C - 90
18
- H2C2O4 x H2O RMM = Ans in C above
-
-
- Table ..........(5mks)
- complete table....... (2mks)
- Conditions
-
- Complete table with 12 readings ....(2 mks)
- Incomplete table with 10 - 11 readings ... (1 ½ mks)
- Incmplete table 7 - 9 readings ..... (1 mk)
- Incomplete table with less than 6 readings ... (0 mk)
- Treat initial values of above 40°C and below 10°C as unrealistic and penalize 1 mk tied to time ie t = 0
- Penalize ½mk for each reading greater 50°C from t = 30 sec to a maximum of 1 mk
- Penalize 1 mk all values given in the table are constant
-
- Conditions
- Use of decimals ...... (1 mk)
Accept whole numbers values or reading to 0 or 5 used consistently. Otherwise penalize fully. - Accuracy .... (1mk)
Compare the candidates initial temperature reading ie, at time t = 0 with the school value and if within ±2 °C award 1mk, otherwise if outised ±2 °C penalize fully. - Trend ....(1mk)
Award the first ½mk for a continuous rise in temparature up to a maximum or constant values followed by a drop.
C.T - 2
Use of decimals - 1
Accuracy - 1
Trend - 1
- complete table....... (2mks)
-
- Graph .. 3mks
marking- Labelling (Both axis)....(½ mk)
Penalties- Penalties fully for inversion of axis.
- Penalize fully for wrong units given, otherwise ignore units if units are omitted.
- Scale .... (½ mk)
Conditions- Area covered by the plots should be at least ¾ of the plotting otherwise penalize fully.
- Intervals must be consistent, otherwise penalize fully.
- Plotting.. (1mk)
Conditions- Award 1mk if at least 9 points correctly plotted.
- Award ½mk if 6 -8 points are correctly plotted, otherwise award 0
- Accept plot even if axes are inverted.
- Shape ...(1mk)
- Award ½mk for a straight lin showing progressive increase in tmeprature.
- Award the other ½ mk for an extrapolated straight lien showing a drop.
Labelling - ½
Scale - ½
Plotting - 1
Shape - 1
03
- Labelling (Both axis)....(½ mk)
- ΔT shown on the extrapolated graph and correct award 1 mk. If not shown but correct award ½mk
- Graph .. 3mks
- Amount of heat
ΔH = MCΔT
= 50 x 4.2 x ΔT
= Ans in Joules
- Table ..........(5mks)
-
a Observation Inference The solid dissolve to form a pale blue solution Soluble compound Cu2+ present b i Observation Inference Blue precipitate formed insoluble in excess Cu2+ present ii Observation Inference Pale blue precipitate formed , soluble in excess to form a deep blue solution Cu2+ present iii Observation Inference pH= 7 The solution is nuetral iv Observation Inference No effervescence , fizzing sound, bubbles produced CO32-, SO32- , HCO3 absent v Observation Inference White precipitate formed insoluble on boiling SO42- Present vi Observation Inference White precipitate formed SO42- Present -
i Observation Inferences The solid dissolves to form a colourless solution Non - polar compound ii Observation Inferences pH 7 The solution is neutral iii Observation Inferences Acidified potassium dichromate (VI) turns from orange to green R-OH present iv Observation Inferences No bubbles , no effervesence , no fizzing sound H+, H3O+, -COOH absent v I Observation Inferences Effrevesence occur , bubbles formed, fizzing sound produced H+, H3O+, -COOH present II Observation Inferences pH= 3 F is stongly acidic
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