Waec 2017 Chemistry (Practical Alternative A) Answers – May/June Expo
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NOTE THAT ^ MEANS Raise to power.
Draw Your Table As Usual And
Input The Following:-
Volume of pipette=25.00cm^3
indicator used- Methyl orange colour change at end point-yellow to orange/purple
Note Use Your School End Point.
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2a)
Tabulate
Test
(i)Fn H2O, then filter
Observation
White residue and blue filtrate was observed
Inference
Fn is a mixture of soluble and insoluble salts
Test
(ii)Filtrate NaOH(aq) in drops, then in excess
Observation
A blue gelatinous precipitate which is insoluble in excess NaOH(aq) was formed
Inference
Cu2 present
Test
(iii)Filtrate NH3(aq) in drops, then in excess
Observation
A pale blue gelatinous precipitate was formed. The precipitate dissolves or is soluble in excess NH3(aq) to give a deep blue solution
Inference
Cu2 confirmed
Test
(iv)Filtrate dil.HNO3 AgNO3(aq)
Observation
No visible reaction White precipitate formed
Inference
Cl- present
Test
NH3(aq) in excess
Observation
Precipitate dissolved in excess NH3(aq)
Inference
Cl- confirmed
2bi)First portion of residue NaOH(aq) in drops, then in excess
Observation
White powdery precipitate which is insoluble in excess NaOH(aq)
Inference
Ca2 present
Test
2bii)Second portion of residue dil.HCl
Observation
Effervescence/bubbles; colourless, odourless gas evolved.
Gas turns lime water milky and turns damp blue litmus paper red.
Inference
Gas is CO2
CO3^2- or HCO3- present
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1)
Tabulate
Burette reading|Final burette reading(cm^3)|Initial burette reading (cm^3)| Volume of acid used(cm^3)|
Rough- 24.10,0.00,24.10
First- 23.80, 0.00, 23.80
Second- 23.75, 0.00, 23.70
Third- 23.75, 0.00, 23.75
Average volume of A used = 23.80 23.70 23.75cm^3/3
=23.75cm^3
1bi)
CAVA/CcVc=2/1
Cc=CAVA/2VC
=0.100*23.75Moldm^-3/2*25.00
=0.0475moldm^-3
amount of A used = 0.100x VA/1000=0.100*23.75/1000 =0.00237
2moles Of A = 1mole of C
0.002375mol of A = 0.002375mol/2
100cm^3 of C contain 0.00237*100mol/2*25 =0
1000cm3ofCcontained0.002375x1000mol
2×25
=0.0475mol
concentration of C in moldm-3 =0.0475moldm-3
1bii)
Molar mass of Bing mol-1:
Molar mass of Na2CO3.yH2O=mass concentration of Bingdm-3
molar concentration of Binmoldm-3
=13.6gdm-3
0.0475moldm-3
=286gmol-1
1biii)
Molar mass of Na2CO3 =[(2×23) 12 (16×3)]=106gmol-1
Mass of anhydrous Na2CO3=106×0.0475gdm-3
=5.035gdm-3
Mass of water=13.6-5.035gdm-3
=8.565gdm-3
Mass of Na2CO3 =Molar mass of Na2CO3
Mass of water y×Molar mass of water
5.035=106
8.565 18y
y =106×8.565
5.035×18
=10
3ai)
lime juice is acidic in nature and the colour of methyl orange in acidic medium is red
3aii)
Iron(iii)chloride will be reduced to iro(ii) with yellow deposit of sulphur
3aiii)
The color of KM2O4 is decolorized because SO2(g) acts as a reducing agent.
3aiv)
Addition of ethanoic acid to KCO3 results to the liberation of a colorless and odorless gas CO2 which turns lime water milky
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