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

Procedure Table 1 ( 5mks)

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
2NaOH + H₂A → Na₂A + 2H₂O
2 moles 1 mole NaOH 23 + 16 + 1 = 40
Moles of NaOH Used = ans
Moles of dibasic acid in 25cm³of solution M = ½ x moles of NaOH Above

r.f.m = 2 + 88 + 18n = ans C( iii)
18n = ans

Sample result

Procedure II

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	SO3^2- SO3^2- CL- SO3^2- 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 )