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WAEC Chemistry Practical Answer – 2020 Expo

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(1a)
In a tabular form
Final |15.25|0.00|45.79|
Initial|0.00||15.25|30.53|
Volume of acid used |15.25|15.28|15.26
Average volume of acid used = 15.25 + 15.26/2
= 15.255cm³
=15.26cm³
(1bi)
Given: Mass of conc of A = 5g/500cm³ = 5g/0.5dm³
Ca = 10g/dm³
A is HNO3
Therefore; Molar mass = 1+14+(16×3)
= 15+48
=63g/mol
Molarity of A = gram conc/molar mass
Ca = 10/63 = 0.1587mol/dm³
(1bii)
Using CaVa/CbVb = nA/nB
With reacting equation:
HNO3 +NaOH–>NaNO3 + H2O
nA = 1, nB = 1
0.1587×15.26/Cb×25.00 = 1/1
25Cb = 0.1587×15.26
CB = 0.1587×15.26/25
CB = 0.09687mol/dm³
(1biii)
B is NaOH
Molar mass = 23+16+1
=40g/mol
Conc of B in g/dm³ = molarity×molar mass
=0.09687×40
=3.8748g/dm³
(1biv)
No of moles present in 250cm³ of NaOH is
= molar conc. × volume
= 0.09687 × 250/1000
= 0.0242 moles
Mole ratio of NaOH and NaNO3 is 1:1
No of moles of NaNO3 which reacted is 0.0242
Mass of NaNO3 formed = molar mass × no of moles
= 85 × 0.0242
=2.057grams
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(2a)
TEST
C + burning splint
OBSERVATION
Sample C bursts into flame
It burns with non-smoking blue flame, without soot.
Colourless gas that turns wet with blue litmus paper faint red and turns lime water milky is present
INFERENCE
C is volatile and flammable. The gas is CO2 from combustion of a saturated organic compound.
(2bi)
TEST
C + distilled water + Shake
OBSERVATION
Clear or colourless solution is observed
INFERENCE
C is miscible with water
(2bii)
TEST
C + Acidified K2Cr2O7
OBSERVATION
Orange color of K2Cr2O7 Solution turns pale green and eventually pale blue on cooling
INFERENCE
C is a reducing agent
(2c)
TEST
D + C + 10% NaOH + Shake
OBSERVATION
D dissolve slowly in C and produces reddish brown solution
Reddish brown solution turns yellow precipitate. The precipitate has an antiseptic odour
INFERENCE
D is soluble in organic solvents
Ethanol, ethanal or a secondary alkanol is present
(2d)
Compound belongs to the class of secondary alkanol
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(3ai)
Zinc nitrate
(3aii)
2 Zn(NO 3 )2 —–>2 ZnO + 4 NO 2 + O 2
(3aiii)
It turns white when cold from it yellow colour when it was hot
(3b)
Pipette / measure 50.0cm3 of the stock solution into a 250
cm3 volumetric flask (containing some distilled water). Shake / swirl and add more distilled water until the mark is reached.
(3c)
Al2(SO4)3 – turns blue litmus red
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