Waec

2018 waec gce chemistry practical answers

2018 waec gce chemistry practical answers

 

*ECONOMICS GCE OBJ*
(1-10)BADCDDABDD
(11-20)CAABDBABAB
(21-30)BADDCBBBCA
(31-40)ACAAADCDCC
(41-50)DAAADCCDBC

Chemistry-Practicals
1ai)
tabulate
Burette reading 1cm^3|1|2|3|
final reading|24.80|34.50|20.65|
initial reading|0.00|10.50|6.55|
volume of acid used 1cm^3|24.80|24.00|24.10|
1aii)
avaerage volume of acide used= VA=24.80+24.00+24.10)cm^3/n =3
VA=72.9/3=24.30cm^3
Hence average volume of A used = 24.30cm^3
1bi)
Concentration of A in moldm^-3
A contains 0.79g of KMnO4 per 250cm³ of solution.
Hence since 250cm³ = 0.79g
100cm³ = Xg
X = 1000×0.79/250 = 3.16g
Hence Conc in g/dm³ of A = 3.16gdm^-³
But molar conc = mass conc(gdm^-³)/molar mass(g/mol)
Hence molar Conc.Of A= 3.16gdm^-3/molar mass ofA
But molar mass of A = KMnO4
= 39 + 55 + 4(16) = 39 + 55 + 64
= 158g/mol
Hence Conc in moldm^-³ of A = 3.16gdm^-3/158g/mol
= 0.020moldm^-3
1bii)
To get concentration of B in moldm^-³, we use the relation
CAVA/CBVB = na /nb
Where,
CA = 0.02moldm^-³
VA = 24.05cm^-³
na = 1
CB = ?
VB = 25.0cm³
nb = 5
0.02×24.05/CB×25 = 1/5
CB = 5 × 0.02×24.05/25
= 2.405/25
=0.0962
CB = 0.096moldm^-³
1biii)
(NH4)2 SO4-FeSO4, 6H2O——>250^2-4 +Fe^2+2NH^+4
N(SO4^2-)/N(Fe^2+)=2/1===>N(SO4^2-)=2*N(Fe^+
C(SO4^2-)=2*0.0962
=0.192moldm^-3
1biv)
m(SO4^2-)=C*M*Vdm^3
SO4^2-
32+16*14=96gmol
=
=0.192*96*250/1000
=4.608
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2)
A is ZnSO4(aq)
B is NaOH(aq)
C is Pb(NO3)2 (aq)
D is HCL(aq)

-Reasons-
A- since it has two no observable reaction and one white ppt
B- Reasons
Since it forms a white precipitate soluble with NaOH
C – Reason’s
Since it has no reaction and forms a white gelatinous precipitate soluble in excess
D- Reasons
– since it gives three white precipitate
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3ai)
Sodium is stored under paraffin oil to prevent its oxidation by atmospheric gases
3aii)
desiccator lid must be greased with a thin layer of grease, to ensure an airtight seal.
3aiii)
Because it is hygroscopic
3bi)
Add an acid eg HCL and this gives a neutralization reaction and the indicator react with the acids. Heat the solution to obtain the NaCL(s)
3bii)
Dip a rod in aqueous Ammonia and dip in a beaker containing the acids, only HCl produces dense white fumes of ammonium chloride
3ci)
i)Beaker
ii)glass rod
iii)volumetric flask
3cii)
On adding the dilute HCL to Na3SO3 a gas with a chocking smell is allow
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