Waec 2017 Chemistry (Practical Alternative A) Answers – May/ June Expo

NOTE THAT ^ MEANS Raise to power.
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
====================
(2a)(i)Fn+H2O,then
filter
White residue and blue
filtrate was observed
Fn is a mixture of
soluble and insoluble
salts
(2b)Filtrate+NaOH(aq)in
drops,then in excess
A Blue gelatinous
precipitate which is
insoluble in excess
NaOH(aq)was formed
Cu2+present
(2bii)Filtrate+NH3(aq)in
drops,then in excess
A pale blue gelatinous
precipitate was
formed.The precipitate
dissolves or is soluble
in excess NH3(aq)to give
a deep blue solution
Cu2+confirmed
(2c)Filtrate+dil.HNO3
+AgNO3(aq)
+NH3(aq)in excess
No Visible Reaction
White Precipitate
formed
Precipitate dissolved in
excess NH3(aq)
Cl-present
Cl-confirmed
(2di)(i)First Portion Of
residue+NaOH(aq)in
drops,then in excess
White Powdery
precipitate which is
insoluble in excess
NaOH(aq)
Ca2+present
(2dii)Second Portion Of
residue+dil.HCl
Effervescence/bubbles;
colourless,odourless
gas evolved.Gasturns
lime water milky and
turns damp blue litmus
paper red.
GasisCO2
CO3
2-orHCO3
-present
3i)Lime Juice is acidic in Nature and the colour of methyl orange in
acidic medium is red
3ii)Iron(II)Chloride will be reduced to Iron(II) with the yellow
deposit of sulphur
3iii)
The color of KM2O4 is decolorized because
SO2(g) acts as a reducing agent.
3iv)Addition of ethanoic acid to k2lo3 results to the liberation of a
colourless or odourless gas co2 Which turns lime water milky

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