Waec 2017 Chemistry Essay, Obj Answers – May/June Expo

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WAEC 2017 CHEMISTRY ANSWER NOW READY
Chemistry obj
1ACDBCBDBCA
11DBAADCDBCB
21DBCBCCDBAD
31BACBACBCCD
41BDACABDABD
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1ai)
an acid base indicator is a weak acid or weak base that exhibits a colour change as the concentration of hydrogen (H^+) or hydroxide (H^-) ions changes in an aqueous solution
1aii)
methyl-orange
1b)
i)oxidizing agent
ii)mixed anhydride
1c)
enthalpy of combustion is the heat evolved when one mole of a substance is completely burns in oxygen gas under standard conditions
1cii)
enthapy of combustion is always negetive since it involvs a rise in temperature and heat of reactants is greater than the heat of products
1d)
primary cell is a cell that cannot be re charged while secondary cell is a cell that can be recharged
1dii)
primary cell — daniell cell
secondary cell — lead accumulator
1e)
mole is the unit of measurement in the International System of Units (SI) for amount of substance. The unit is defined as the amount of a chemical substance which contains as many representative particles, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12 (12C), the isotope of carbon with relative atomic mass
1f)
n= V=40cm^3
c= 0.40moldm^3
n=cv/1000
0.40*40/1000 = 0.016mol

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 acrose 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)
R.A.M=(75/100*35)+(25/100*37)
=2634.25
2di)
I CO2 by downward delivery
II HCL by downward delivery
III H2 by upward delivery
2dii)
I because it is denser than air
II because it is denser than air
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
5ai) worm 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.

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5iia) Sodium chloride is used for:
=preserving food
=glazing pottery

5iib) 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)

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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.
GALANTWAP FOREVER

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