# CHEMISTRY PAPER 3 Marking Scheme - 2017 MURANG'A MOCK EXAMINATION

1. Procedure Table 1 ( 5mks)
1. Complete table   -   1mk
Complete table with 3 titrations done – 1mk
Incomplete table with 2 titrations done    ½ mk
Penalties
wrong subtraction
Inverted table
Burette readings beyond  50cm3  unless explained
Unrealistic titre values eg less 1cm3  or in hundrends
Pennalise  ½mk to  Maximum  ½mk

Use of decimals – ( Tied to first and second rons  only) 1mk
Accept one or two decimals used consistency otherwise penalize  fully ( A ward zero marks)
Accept inconsistency  of use of zero’s on the initial burette readings eg   0, 0.0, 0.00.
If two decimals, use the second digit as 0. Or 5 otherwise penalize fully.

Accuracy – 1mk
Compare the candidates titre values with the school value and award as follows.
1. If at least one is within  t  0.1 of S.V – 1mk
2. If none but at least one within t 0.2 of S.V – ½ mk
3. If none within  t  0.2 of S.V award  0 mk.

Principle of averaging -  1mk
1. 3 Titrations done, 2 are consistent – 1mk
2. If only 2 titrations done, are consistent – 1mk

Final Accuracy  - 1mk
tied to correct average of titre  - 1mk
Compare the candidate correct average titre with S.V . award as follows.
1. If  within  t 0.10cm3 of S.V  - 1mk
2. If not  t  0.10  but within  t 0.20 of S.V   ½ mk
3. If not within  T 0.20 cm3 of S.V 0mk

2. 2NaOH + H2A → Na2A + 2H2 O
2 moles  1 mole  NaOH 23 + 16 + 1 = 40
Moles of NaOH  Used = ans
Moles of dibasic acid in  25cm3 of solution M = ½  x  moles of NaOH  Above

3.
1. 250cm3 of M= ans ( b) mole ratio 1: 2
250cm3 of M  ½  X 0.0025 = ½  mk
2. Answer in C(i) X 1000
3. 6.3/answer in C(ii) ½  ans   ½
4. r.f.m = 2 + 88 + 18n = ans  C( iii)
18n = ans

Sample result
 F.B.R 18.9 18.8 18.7 T.B.R 0 0 0 Vol. of soln B used 18.9 18.8 18.7

Procedure II
1. Complete table with 10 readings ( 3mks)
Incomplete table with 8  or 9  readings ( 2mks)
Incomplete table with 5 or 6 readings ( 1mk
Less than 2 readings  ( 0 –mk)
Penalise ½ mk for incorrect 1/t   or value rounded to less than 3 d.p. unless exact ( maximum   penaly 1mk)

Use of decimal (½  mk)
should be whole number or 2 d.p consistent otherwise penalize fully.

Accuracy – 1mk
Compare the first record at 40o C of S.V  to candidates readings.
H + 2o C award 1mk otherwise penalize fully

Trend  -  ½  mk
Time progression should be consistent or continuous drop from 40o   to 80o

Graph -  3mks as shown
Labelling of axes ( ½ mk)
Scale  ( ½  mk)
Area covered by the graph ( plots)  should be at least half of the grid provided.
scale interval must be consistent on each  axis.
Plotting – ( 1mk)
3 – 4 [points correctly plotted award   -  1mk
2 points are correctly plotted ½  mk
Mark all points plots with a tick   or cross
Line/ shape of graph- ( 1mk)
Accept a correct line passing through at least 2 correctly- plotted point and origin. 1mk
2. Calculation for  time at  65o C from graph time = Reciprocal of  1/t .  1mk

3. the rate of reaction increases as the temperature increases due to K..e increases and collisions.

QUESTION 2
 Observations Inferences a)Solid melts  ½   Red litmus paper ½  turns blue. Blue litmus paper  turns red. Colourless liquid seen in cooler part of test tube. NH4 + ions  ½   present / NH3  gas evolved.Acidic  ½  gas evolved  hydrarted salt. b) Solid Q dissolves to form Colourless solution ½ Ionic compound / soluble salt /polar ½ ii) White ppt is formed SO32-    SO32-   CL-   SO32-   present, iii)  White ppt formed insoluble in acid SO4 2-  Pb2+  ,Al 3+  ions iv) White ppt formed 1 dissolves in excess to form colourless soln Zn2+  Pb2+ ,AL3+ ions present v)White ppt formed  Insoluble in excess  ½ mk Pb2+ , Al 3+  ions present vi) white ppt formed ½  dissolved on warming Pb2+ ions present  ½ 3.a) Soofty  flame burns with smoky flame ½ b) Dissolve to form a colourless   generous solution Polar organic compound ½ c.i) No observable change  1mk) R- OH  absent ii) Effectivescene / bubbles seen R –COOH  or iii) Purple acidified KMnO4 Decolourised ( Turns colourless )

#### Download CHEMISTRY PAPER 3 Marking Scheme - 2017 MURANG'A MOCK EXAMINATION.

• ✔ To read offline at any time.
• ✔ To Print at your convenience
• ✔ Share Easily with Friends / Students

### Related items

.
Subscribe now

access all the content at an affordable rate
or
Buy any individual paper or notes as a pdf via MPESA
and get it sent to you via WhatsApp