Chemistry Paper 3 Marking Scheme
KCSE CLUSTER TESTS 15
Chemistry Paper 3
Volume of pipette. 25.0 cm3 Penalize ½ mark if not in decimal Table 1 (3marks)
— Complete table 1 mark√1
Conditions:
-Complete table with 3 titration 1 mark√1
-Incomplete table with 1 titration = 0 mark
— use of decimals (tied to rows 1 and 2 only ) = 1 mark)√1
Conditions:
-Accept only 1 to 2 decimal place used consistently; otherwise penalize fully.
-Accept 2 decimal place if the second dp is ‘0’ ‘5’ otherwise penalize fully.
-Accept inconsistency use of zeros as initials volume i.e 0,0.0,0.00.
— Accuracy = 1 mark (only tick the correct value otherwise don’t tick);
-Compare the candidates /students value with the school value (S.V): and tick (√) the chosen value where it earns a mark.√1
Conditions:
-If no value is within 0.20 cm3 of SV = 0 mark.
Note:
-If there is wrong arithmetic//subtraction in the table compare the SV with the worked out Correct value// the titre and award accordingly.
— principal of averaging = 1 mark√1 values averaged MUST be shown and MUST be within ±0.20 cm3 of each other,
Conditions:
-If three consistent values are averaged. = √1
-If 3 titrations are done but only 2 are within 0.20cm3 of each other rare averaged = 1 mark.
-If only 2 titrations are done, are consistent and are averaged =½ mark.
-If only 2 titrations done, are inconsistent and get averaged = 0 mark.
Penalties:
-Penalize ½ mark for wrong arithmetic error outside +2 units in the second decimal place.
-Penalize½ mark 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 answer to 2 dp. Otherwise penalize ½ mark for wrong rounding off to whole numbers.
Note
-Accept answer if it works out exactly to 1 dp as to a whole number and credit fully.
— Final answer (tied to correct average titre) = 1 mark.√1
-Compare the students//candidates correct average titre to the SV then:
Note
-If wrong values are averaged or when there are no options for averaging pick the correct Values (if any) following the principle of averaging average and award accordingly.
Examples
-For 24.9, 24.7 and the SV is 24.35 pick 24.6 and 24.7.
-For 24.2,24.4 and 24.3 pick all.
-For 25.3,25.1 and 25.5 and the SV is 24.5 pick either 25.3 and 25.1 or 25.3 and 25.5 depending in the SV but not all.
-For 24.6,24.3 and 24.9 none to be picked for averaging and award 0 mark for final answer.
Procedure II
Calculations:(Questions and calculations)
(a) The average titre = 25.0 cm3
Principles of averaging = 1 mark
-Values averaging MUST be shown and MUST be within ±0.20cm3 of each other.
Conditions:
-If three consistent values are averaged = 1 mark√1
-If 3 titrations are done but only 2 are within 0.20 cm3 of each other are averaged = 1 mark√1
-If only 2 titrations are done, are consistent and are averaged = ½ mark.
-If 3 possible but only 2 re-arranged = 0 mark.
-If 3 titrations are done, are inconsistent and yet are ALL averaged = 0 mark.
-if only 2 titrations done, are inconsistent and yet averaged = 0 mark.
Penalties:
-Penalise½ mark for wrong arithmetic error outside +2 and in the second decimal place.
-Penalize ½ mark 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 answer to 2 decimal places; otherwise penalize ½ mark for wrong rounding off and 1 dp or to whole numbers.
Note:
-Accept answers if it works out exactly to 1 dp a whole number and credit fully.
(b). Moles of HA
Conditions and penalties
-Penalize ½ mark for wrong transfer of molarity otherwise penalize fully.
-Accept rounding of answer to at least 3 dp. otherwise penalize ½ mark.
-Penalise ½ mark for wrong answer if the arithmetic error is outside ±2 units in the 3 dp.
-Units given must be correct; penalize ½ mark for wrong units.
(d). From moles ration: 1 mole of acid reacts with 1 mole of alkali;
Conditions and penalties:
-Penalize ½ mark for wrong transfer of moles, otherwise penalize fully.
-Accept rounding off of answers to atleast 3 dp otherwise penalize ½ mark.
-Penalise ½ mark for wrong answer if the arithmetic error is outside ± 2 units in the 3 dp.
-Units given must be correct; penalize ½ mark for wrong units..
(ii). Mass = moles /litre x RMM.
Conditions and penalties:
-Penalize½ mark for wrong transfer of molarity and mass; otherwise penalize fully.
-Accept rounding off of answer to at least 3 dp. otherwise penalize ½ mark.
- Penalize ½ mark for wrong answer if the arithmetic error is outside ± 2 units in the 3 dp.
-Units given must be correct. Penalize ½ mark for wrong units.
(f). Concentration of XCl
Total mass – mass of KoH
8.4g – 5.6 = 2.8g/litre.
Conditions and penalties:
-Penalize ½ mark(½√) for wrong transfer of answers in (ii) above. Otherwise penalize fully.
-Accept rounding off of answers to atleast 3 dp. otherwise penalize √½ mark.
-Penalize ½ mark for wrong answers if the arithmetic error is outside ±2 units in the 3 dp.
-Units given must be correct; penalize ½ mark for wrong units.
(g). Percentage
Conditions
-Penalize ½ mark for wrong transfer of answers in f above. otherwise penalize fully.
-Accept rounding off to answer to atleast 3 dp. otherwise penalize ½ mark.
-Penalize ½ mark for wrong answer if the arithmetic error is outside ± 2 units in the 3 dp.
-Units given must be correct; penalize ½ mark for wrong units.
22 marks
Complete table.
(a). Complete table√1
Conditions:
Complete table with 3 titration = √1
Incomplete table with 2 titration =√½
Incomplete table with 1 titration = 0 mark
Penalties
-Wrong arithmetic// subtraction.
-Inverted tables.
-Burette readings beyond 50.0 cm3, unless explained.
-unrealistic litre values i.e too low (below 1.0 cm3) or too high (100cm3)
Note
-Penalized √½ mark EACH to a maximum penalty of √ mark (i.e) penalize ½ mark only once.
(b). Use of decimal (tied to rows 1 and 2 only) = 1 mark.
Conditions
-Accept only 1 to 2 dp used consistently; otherwise penalize fully.
-Accept 2 dp only if the second dp is ‘0’ ‘5’ otherwise penalize fully.
-Accuracy = 1 mark (only tick the correct value i.e 0,0.0,0.00
(c). Accuracy = 1 mark (only tick the correct value otherwise don’t tick.
-Compare the candidates//students value with school value (SV), and tick (√) the chosen value where it earns a mark.
Conditions
-If any value is within ± 0.10 cm3 of SV award √1
-If no value is within ± 0.10 cm3 but atleast one is within 0.20 of SV =½ mark (√½)
-If no value is within 0.20cm3 of SV award zero mark (0 mark)
Note:
If there is wrong arithmetic //substracha in the table compare the SV with the worked out correct value// the tilre and award accordingly.
(b).
i. N
ii. R
iii. P
iv. M
(c). (i). Sample M
9 marks
9 marks