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Paper III: Practical – Chemistry
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2020-WAEC-GCE-Chemistry-Practicals-ANSWERS
(1a)
Mass number A = 1.85g
Vol of A = 250cm³
Mass of B =400g in 1dm³
Average title = 25.00+24.90/2
=49.90/2=24.95cm³
Edited.
(1b)
(i) pink
(ii) colourless
(1ci)
Conc of B in mol/dm³ ; ?
Mole =4.00/40 =0.1mol
Mole = conc * vol(dm³)
Conc =mole/vol
Conc =0.1/1
Conc=0.1mol/dm³
Conc of B =0.1mol/dm³
(1cii)
Cb=0.1mol/dm³ , Vb=25.0cm³ , Ca =? Va=24.95cm³
na=1, nb=1
CaVa/CbVb
Ca*24.95/0.1*25.0=1/1
Ca=0.1*25.0/24.95=2.5/24.95=0.100mol/dm³
(1di)
Molecular mass=?
Mass conc =1.85/250*1000
=7.4g/dm³
Conc(mol/dm³)=mass conc/molar mass
0.1=7.4/molar mass
Molar mass =7.4/0.1= 75g/mol
(1dii)
A=CnH²n+1COOH
CnH²n+1COOH=74
(12*n)+(1x(2n+1)+12+16+16+1=74
12n+2n+1+45=74
14n+46=74
14n=74-46
14/14=28/14
n=2
C²H⁵COOH
(1e)
This is because it is a reaction between weak acid and strong base
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(2a)
TEST
C(aq) + HCl
OBSERVATION
No gas evolved/No observable reaction
INFERENCE
SO²,⁴^- Some may be Present
(2b)
TEST
C+BaCl² + dili H²SO⁴
OBSERVATION
White Precipitate Formed Precipitate insoluble remain Undissolved
INFERENCE
SO²,⁴^- confirmed
(2ci)
TEST
C+NaOH(aq) In drop them in excess
OBSERVATION
No precipitate / No visibie reaction
No Observation reaction.
INFERENCE
Probably NH+⁴ Present
(2cii)
TEST
mixture in c(i) warmed
OBSERVATION
Gas is evolved
Gas is basic
Gas has a Pungent or smell
INFERENCE
NH₃ gas evowed hence NH³ presert
(2di)
TEST
C+HNO³ + AgNO³
OBSERVATION
No observable reaction
No gas evolved
INFERENCE
C1^- Present
(2dii)
TEST
Precipitate + HNO³
OBSERVATION
White precipitate
Precipitate insolubie
INFERENCE
C1^-confirmed
(2ei)
(i)(NH⁴)²SO⁴
(ii)NH⁴Cl
(2eii)
(i)Ammonium teraoxosulphate(iv) salt(NH⁴)²SO⁴)
(ii)NH⁴CI > Ammonium chloride
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(3ai)
There will be evolution of SO²
(3aii)
a green color is observed
(3aiii)
There is effervescence I:e CO² is liberated
(3b)
(i)HCl
(ii)Zn(OH) will dissolves readily while (⁴(OH)² will not.
(3c)
(i)it reduce the mass of the salt
(ii)to purity it