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WAEC 2017 POSTED - CHEMISTRY ANSWERS

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WAEC - CHEMISTRY

EXAM TIME: Tuesday, 18th April, 2017

Chemistry 2 (Essay) 2.00pm – 4.00pm
Chemistry 1 (Objective) 4.00pm – 5.00pm
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CHEMISTRY ESSAY/OBJ ANSWERS

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COMPLETED
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CHEMISTRY OBJ:
1ACDBCBDBCA
11DBAADCDBCB
21DBCBCCDBAD
31BACBACBCCD
41BDACABDABD


(1ai)
Acid base indicator is a weak complex organic acid or base which gives different colours in acidic or basic medium

(1aii)
-Methyl Orange
-Litmus
-Phenolpthalein

(1bi)
-4Cu+2NO2->4CuO+N2=Oxidizing agent
-H20+2NO2->HNO3+HNO2=Oxidizing agent

(1ci)
Enthalphy of combustion is the energy evolved when one mole of a substance burns completely in excess oxygen

(1cii)
Because heat is always evolved

(1d)
-Primary cell cannot be recharged after use but a secondary cell can be recharged after use
-Primary cells are irreversible while secondary cells are reversible
eg of primary cells are
-Daniel cell
-Leclanche cell
eg of secondary cell are
-Fuel cell
-Lead-acid accumulator

(1e)
A mole is the amount of a pure substance containing the same number of chemical units as their atoms is exactly 12g of carbon -12 (ie 6.02*10^23)

(1f)
n=?
V=40cm^3
C=0.40mol/dm^3
n=CV/1000
=(0.40*40)/1000
=0.016mol

(1g)
Carbon or graphite
Plantinum

(1h)
-induced dipole - induced dipole interaction or dispersion force
-dispole- dispole interaction

(1i)
-carbon(ii)oxide and water or co and H2O

(1j) 
26FE³+ -- 1s²2s²2p^6 3s²3p^63d³4s²

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(2a)
(i)periodic law states that properties of elements are the periodic function of their atomic number
(ii) periodic property of element is the variation of the properties of element in a regular pattern both down the group and across the period

(2aii)
(i)ionization energy
(ii) electron affinity
(iii) electropositivity

(2aiii)
-ionization energy increase from left to right acrose a period because the nuclear charge increase progressively while the screening effect remain constant
-atomic reduis decreases from left to right acrose the period because nuclear charge effect out weighs screening effect, which remains constant because the number of shell in a period is constant
-electron affinity increase from left to right across a period because an additional electron will experience a greater repulsion than altraction from the positively charged nucleus

(2b)
the relative atomic mass of an element is the mass of one atom of the element compared with the mass of an atom of the carbon - 12 isotope whos mass is exactly 12

(2bci)
isotopy

(2cii )
the isotopes have difference number of neutron

(2ciii )
(35)(17)Z=75%, (37)(17)Z=100-75=25%
Ar=(355*37+37*255)/100
=2625+9255/100
=3550/100
=35.5

(2di)
-CO 2 by downward delivery
-HCL by downward delivery
-H2 by upward delivery

(2dii)
-because CO2 is denser than air
-because HCl is denser than air

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(3ai)
Thermal cracking

(3aii)
C4H8

(3aiii)
(I) H H. H
| | |
H-C-C-C=C
| | | |
H H H H
and
(II) H H
| |
H-C-C=C-C-H
| | | |
H H H H

(3aiv)
I-normal butene
II -But-2-ene
Ni

(3av)
C4H8 +H2 -------> C4H10
250c

(3b)
METHOD:
-rub the fat on a blank white paper
-pour Sudan (III) dye over the paper
-wash the paper under tap

OBSERVATION:
-rubbing the fat on the paper produces a translucent color
-pouring sudan (III) will stain the blot red

CONCLUSION:
-fat is present because sudan (III) can stain fat red

(3c)
Given 2S02 + 02 + 2 H 20 - - - > 2H2SO4
1 mole produce 2 mole
1.3 mole produce 3 mole
moles of H 2 SO4 = 3 mole
molar mass of H2SO4 = (1+2)+ 32 + (4*16)
= 2+32+64
= 98 gmol-^1
mole = mass/molar mass
mass and H2SO4 = Mole*molar mass
= 3*98
= 294 g

(3di )
DH2 

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(5ai)
Warm the unknown substance with concentrated H2SO4 in a test tube. if trioxonitrate(v) ion's are/is present, some trioxonitrate(v) acid will be condensed as oily drops on the upper part of the tube. On further heating, the acid decomposes yielding brown fums of NO2 gas.

(5aii)
(i) Sodium chloride is used for:
-preserving food
-glazing pottery

(ii) Sodium trioxocarbonate(iv) is used for:
-standardizing acids in the laboratory
-the manufacture of glass

(5bi)
C(s) + O2(g) =====> Co2(g)
Co2(g) + C(s) =====> 2Co(g)
Fe2O3(s) + 3Co(g) =====> 2Fe(l) + 3Co2(g)
The limestone present decomposes as follows:
CaCo3(s) =====> CaO(s) + Co2(g)
Which is available to remove silicon(iv) oxide impurity
SiO2(s) + CaO(s) =====> CaSiO3(s)

(5bii)
Faraday's first law of electrolysis states that, the mass (m) of an element discharged during an electrolysis is directly proportional to the quantity of electricity (Q) passing through it.












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