2020 WAEC Chemistry Practical Expo Questions and Answers

2020 WAEC Chemistry Practical Expo Questions and Answers

    EXAM SCHEDULE

Tuesday August 18th, 2020

9:30 am

SOLUTIONS

Verified WAEC 2020 CHEMISTRY PRACTICAL ANSWERS
.

(1a)
Burette reading (cm)³|1st reading|2na Reading|3rd reading|
Final |15.25|30.53|45.79|
Initial |0.00|15.25|30.53|
Volume of acids used |15.25|15.28|15.26

Average volume of acid used =15.25+15.26/2
=15.255cm³
=15.26cm³
Or =15.27cm³

(1b)
Given; mass con of A =5g/500cm³ = 5g/0.5dn³
CA=10g/dm³
A is HNO3
Therefore molar mass =1+14+(16*3)=15+48=63g/mol¹

Molarity of A = gram con/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
=0.0242moles
Mole ratio of NaOH and NaNO3 is 1;1
No; of mole of NaNO3 which reacted is 0.0242
Mass of NaNO3 formed =molar mass * no; of moles
=85*0.0242
=2.05grams
{
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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
==============================
===

*3ai)*
oxygen

*(3c)*

It turns blue litmus paper red.

*3d*
Pure copper(II) sulfate is white. It is also known as anhydrous copper(II) sulfate because it has no water in it. When water is present in a sample of copper(II) sulfate it turns blue. … This colour change can be used to detect the presence of water (or water vapour).

*3c(i)*
When exposed to light, silver chloride decomposes into gray metallic silver and chlorine. The light sensitivity of the silver chloride and other silver halides, such as silver bromide and silver iodide, forms the basis of the photographic process.

*3bi(.)*

Phenolphtalein is chosen because it changes color in a pH range between 8.3 – 10. It will appear pink in basic solutions and clear in acidic solutions. ..

*3c(i)*
When exposed to light, silver chloride decomposes into gray metallic silver and chlorine. The light sensitivity of the silver chloride and other silver halides, such as silver bromide and silver iodide, forms the basis of the photographic process.

*3cii*
The ammonia combines with silver ions to produce a complex ion called the diamminesilver(I) ion, [Ag(NH3)2]+. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.

*3d*

Pure copper(II) sulfate is white. It is also known as anhydrous copper(II) sulfate because it has no water in it. When water is present in a sample of copper(II) sulfate it turns blue. … This colour change can be used to detect the presence of water (or water vapour).

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