2020 WAEC-CHEMISTRY-PRACTICAL-ANSWERS

WAEC-CHEMISTRY-PRACTICAL-ANSWERS

Pls use ur school end points.

(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 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

(2C)

TEST: D + C + 10% NaOH + Shake

OBSERVATION: D dissolves 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

(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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Complete.

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Posted by CLEF MIKE on August 18, 2020.

Categories:WAEC