2020 Waec Chemistry Practical Answers

2020 Waec Chemistry Practical Answers

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In a tabular form

Burette Reading|1st reading|2nd reading|3rd reading
Final |15.25|0.00|45.79|
Volume of acid used |15.25|15.28|15.26

Average volume of acid used = 15.25 + 15.26/2
= 15.255cm³

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

Molarity of A = gram conc/molar mass
Ca = 10/63 = 0.1587mol/dm³

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

B is NaOH
Molar mass = 23+16+1
Conc of B in g/dm³ = molarity×molar mass

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
C + burning splint

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

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

C + distilled water + Shake

Clear or colourless solution is observed

C is miscible with water

C + Acidified K2Cr2O7

Orange color of K2Cr2O7 Solution turns pale green and eventually pale blue on cooling

C is a reducing agent

D + C + 10% NaOH + Shake

D dissolve slowly in C and produces reddish brown solution

Reddish brown solution turns yellow precipitate. The precipitate has an antiseptic odour

D is soluble in organic solvents

Ethanol, ethanal or a secondary alkanol is present

Compound belongs to the class of secondary alkanol
Zinc nitrate

2 Zn(NO 3 )2 —–>2 ZnO + 4 NO 2 + O 2

It turns white when cold from it yellow colour when it was hot

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.

Al2(SO4)3 – turns blue litmus red


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