WAEC Chemistry Practical Solution Questions and Answer – AUG/SEPT 2020 Expo Runz.

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WAEC 2020 CHEMISTRY PRACTICE ANSWER



(1a) 
In a tabular form

Burette Reading|1st reading|2nd reading|3rd reading
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
===========≈=====================
(2a)
TEST: C+burning splint

OBSERVATION: Sample C burst into flame .It burns with non smoky blue flame without soot. Colorless gas that turns wet blue litmus paper faint red and turns like water milky is present.

INFERENCE: C is volatile and flammable. The gas is CO2 from combustion of saturated organic compound.

(2bi)
TEST: C + distilled water + shake

OBSERVATION: Clear or colorless solution is observed

INFERENCE: C is miscible with water

(2bii)
TEST: C + Acidified K2Cr207

OBSERVATION: Orange color of K2Cr207 solution turns pale green and eventually pale blue on cooling

INFERENCE: C is a reducing agent

(2d)
Compound belongs to the class of secondary alkanol

=================================
(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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Complete.

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