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NABTEB NOV/DEC GCE 2021 FREE CHEMISTRY PRACTICAL (CHEM) QUESTION AND ANSWER ROOM

Wednesday 10th February, 2021
Alternative to Chemistry Practical (9:00am-11:00am)

### NABTEB NOV/DEC GCE CHEMISTRY PRACTICAL ANSWERS 2021:

#### A. O'LEVEL CANDIDATES

(1)
TABULATE
Final reading: 24.50| 48.80 | 25.60
Initial burette readings: 0.00| 24.40 | 1.00
Volume of acid used: 24.50| 24.40 | 24.60

(1ai)
Volume of acid used = Final burette readings - Initial burette readings

1st titration = 24.50cm³ - 0.00cm³ = 24.50cm³

2nd titration = 48.80cm³ - 24.40cm³ = 24.40cm³

3rd titration = 24.50cm³ - 1.00cm³ = 24.60cm³

(1aii)
Average titre value = 1st + 2nd + 3rd/3
24.50 + 24.40 + 24.60/3
73.50/3 = 24.50cm³

(1bi)
MA = 0.08mol/dm³, VA = 24.5cm³, MB =? , VB = 25.0cm³, NA= 1, nB= 2
MAVA/MBVB = nA/nB
0.08×24.50/25MB = 1/2
25MB = 3.92
MB = 3.92/25 = 0.157
MB = 0.157mol/dm³

(1bii)
Mass concentration = Molar concentration × Molar mass
Molar concentration of acid = 0.08mol/dm³, Molar mass = 2×1+3+32+16×4 = 98g/mol
Mass concentration = 0.08 × 98 = 7.84g/dm³

(1biii)
Molar mass = Mass concentration/Molar concentration
Molar mass = 0.08/0.157 = 56g/mol

(1biv)
YOH = 56
Y + 16 + 1 = 56
Y + 17 = 56
Y = 56 - 17 = 39g

(1bv)
Methyl orange

(1c)
(i) Air bubbles in the burette
(ii) No indicator is use
(iii) Funnels should be removed before titration starts to avoid droplets
(iv) incorrect of reading and scale
=============================

(2a)
=TEST=
(i) C + NaOH (aq) in drops then in excess
(ii) C + NH3(aq) in drops then in excess
(iii) C + BaCl(aq) then followed by dil.HCl

=OBSERVATION=
(i) Blue gelatineous precipitate
(ii) Precipitate dissolves in excess white precipitate
(iii) Precipitate Insoluble

=INFERENCE=
(i) Cu²+ present
(ii) Cu²+ present
(iii) SO4²+ present

(2b)
=TEST=
(i) D + H2O
(ii) Solution of D + NaOH2(aq) in drops then in excess
(iii) Solution of D + NH3(aq) in drops then in excess
(iv) Solution of D + FeSO4(aq) then followed by H2SO4(aq)

=OBSERVATION=
(i) D dissolves in H2O
(ii) White gelatineous precipitate dissolves in excess
(iii) White gelatineous precipitate dissolves in excess
(iv) A brown ring is observed in the rest tube

=INFERENCE=
(i) D is a soluble salt
(ii) Zn²+ present
(iii) Zn²+ present
(iv) NO3+ present
NO3- Confirmed

=============================

(3ai)
P - sulphur
Q - Sulphur (iv) oxide or SO2
S - Sulphur trioxide or SO3
T - Barium tetraoxosulphate (iv) or BaSO4

(3aii)
R could be vanadium (v) oxide or platinum
2SO2(g) + O2(g) ⇌2SO3(g)

(3aiii)
No, this is because hydrochloric acid cannot dissolve tetraoxosulphate (iv) acids from its salt

(3bi)
A black coloration is observed due to the formation of lead (ii) sulphide
Pb (CH3COO)2 + H2S ⇌ Pbs + 2CH3COOH

(3bii)
A yellow deposit of sulphur is observed due to a redox reaction
2H2S + SO2 -------> 3S + 2H2O
=============================================

(1ai)
Experiment 1: Fill the bottle with sodium thiosulphate and by means of pipette, measure about 20ml of potassium iodate. After allowing about 5ml of sodium thiosulphate, add starch indicator of the acidified potassium iodate and continue with titration. Stop the titration after observing a colour change. Record the reading as final burette reading. The initial burette reading us the level of sodium thiosulphate before titration.

Experiment 2: The titration process in experiment 1 is also repeated here but with more precision in terms of taking readings.

(1aii)
Deep blue to light yellow

(1aiii)
This can be as a result of parallax error or decrease in the concentration of the acid
=================================

(2a)
=TEST=
A little of R w as heated in a test tube. Any gas given off was tested damp pH indicator paper

=OBSERVATION=
A white solid formed at the top of the test tube. Pungent gas evolved, pH 8-10

(2b)
=TEST=

=OBSERVATION=

(2c)
=TEST=
Appearance of salt z

=OBSERVATION=
Green in colour

(2d)
=TEST=
Salt z was dissolved in distilled water in a test tube. The solution was divided into two portions
(i) To the first portion, excess sodium hydroxide was added
(ii) To the second portion, a few drops of nitric acid was added followed by aqueous silver nitrate

=OBSERVATION=
(i) A green precipitate insoluble in excess is formed
(ii) A white precipitate is formed
=================================

(3a)
Desiccator

(3b)
[https://i.imgur.com/vrFXUaK.jpg]

(3c)
(i) Ammonia
(ii) Ammonia
(iii) Hydrogen chloride
(iv) Ammonia
=================================

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