Solved Chemistry 9th 2013 paper Mardan Board.
Section B (Marks 32)
Q. 2 Answer any eight parts. Each part carries 4 marks
(i) Differentiate between atomic number and mass
number.
Atomic Number
Mass Number.
1.Definition. The sum of the
1. Definition:
The number of protons in the number of protons and
neutrons present in the
nucleus or The number of
electrons present around the nucleus of an atom is called
mass number.
nucleus of neutral atom is
called its atomic number.
2.Representation. Mass
2.Representation:
Atomic number is represented number is represented by ‘A’
by capital letter “Z”.
3.Mathematically
3.Mathematically
Mass number (A) =
Atomic number = Number
Number of protons (Z) +
of protons
number of neutrons
OR
Z = A - neutrons
4. It does not determine the
identity of an element but
4.It determines the identity
isotopes
of an element
5. Examples:
5. Examples:
The atomic number of carbon The mass number of carbon
is 12.The mass number of
is 6.The atomic number of
oxygen is 16
oxygen is 8
Q 2 (i) Define shielding effect. Does it vary in period?
The reduction in the force of nuclear attraction on the outer
electrons due to inner electrons is called shielding effects or
screening effect. OR the reduction in force of attraction by the
shells present between nucleus and valence electrons is called
shielding effect.
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In shielding effect, the core electrons block valence electrons
from the nuclear attraction.
The shielding effects remain constant from left to right in
periods because the number of shells remains constant.
Q 2 (iii) Discuss the salient features of Dalton atomic theory.
Dalton atomic theory silent features or postulates.
The main postulates of Dalton atomic theory are
a. Matter is composed of very small particles called atoms
b. Atom is indivisible
c. Atoms of the same elements are identical in all respect
like size, shape and mass etc but different from atoms of
other elements.
d. Atoms combine with other atoms by simple whole number
ratio to form compound.
Q 2 (iv) How many elements are there in 7th period? Also
write their names.
Ans
There are five elements in 7th group which are collectively
called halogens and mean salt former. The five elements of the
7th group are fluorine (F), chlorine(Cl), bromine(Br), iodine(I)
and astatine(At).
Q. 2(v) In CaCl2 describe the bond between Ca and Cl
Ans.
In CaCl2, the bond between calcium and chlorine is ionic bond
because electronegativity difference between Ca and Cl is
greater than 1.7 (3.16-1 = 2.16)
Calcium transfers its two valence electron to two chlorine
atoms. As a result double positive charge appears on calcium
and negative charge on chlorine. The opposite ions are held
together by electrostatic force called ionic bond.
Ca (2, 8, 8, 2) ------ Ca 2+ (2, 8, 8) + 2e
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2Cl (2, 8, 7) + 2e -- 2Cl- (2, 8, 7)
Ca 2+ + 2Cl-  CaCl2
Q. 2 (vi) Define allotropy. Write names of allotropic forms
of carbon.
Allotropy:
Definition: The existence of an element in more than one
crystalline form is called allotropy
OR
Different crystalline forms of the same elements which have
same chemical properties but different physical properties are
called allotropes and phenomena are called allotropy.
Example. Crystalline carbon exists in three allotropic forms
diamond, graphite and Bucky Ball.
Q 2 (vii) Write names and symbols of first group elements
Ans
First group elements are called alkali metals which are six in
numbers and given below.
Lithium ( Li), sodium (Na), Potassium (K), Rubidium (Rb),
Cesium (Cs) and Francium (Fr).
Q. 2 (viii) Define isotopes. Explain isotopes of hydrogen.
Isotopes
Atoms of the same elements having same atomic number but
different mass number are called isotopes of that element
(OR)
Atoms of the same element, which have same number of
protons but different number of neutrons in nucleus are
called isotopes. -
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Example. Isotopes of Hydrogen: Hydrogen has three isotopes
and represented by
From above figure, it is clear that all three forms (protium,
deuterium and tritium) have same number of proton and
electron but different number of neutrons. Ordinary
hydrogen is called protium because contains only one proton
but no neutron. Deuterium has one proton and one neutron in
nucleus while tritium has one proton and two neutrons in
nucleus.
Q 2 (ix) What is molarity?
Molarity: The number of moles of solute present in 1litre
(1dm3) solution is called molarity OR the number of moles
of solute dissolve in one litre solution is called molarity.
Mathematically:
𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇𝒎𝒐𝒍𝒆𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆
Molarity = 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝒊𝒏 𝒍𝒊𝒕𝒓𝒆
But
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 𝒊𝒏 𝒈𝒓𝒂𝒎𝒔
Mole = 𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆
𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 𝒊𝒏 𝒈𝒓𝒂𝒎𝒔
So Molarity = 𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 𝑿 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝒊𝒏 𝒍𝒊𝒕𝒓𝒆
Example: When 40 gram NaOH presents in one litre solution
then the solution will be 1Molar NaOH solution.
Q. 2 ( xi) What is electroplating? What is its significance?
Ans. Electroplating:
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Definition: The process in which a thin layer of one metal is
deposited on another metal by electrolysis is called
electroplating.
Examples: zinc plating, chrome plating, silver plating and
tin plating etc
Purpose of electroplating or singnificance:
 Protection of inner metal from atmospheric effect like
corrosion
 Repairing of broken parts of machinery by electroplating
 Decoration of metal by noble metal like silver, gold etc
Section C
Q. 3 a) Explain Neil Bohr’s model of atom.
Ans. Neil Bohr’s atomic theory.
In order to remove the defects of Rutherford atomic model
that why electrons do not fall in the nucleus, Neil Bohr’s
in 1913 gave his atomic model and the main points are
given below:
1. Electrons revolve around the nucleus in a fixed circular
path called orbit or energy level. Each orbit has a fixed
size, shape and energy.
2. Electron does not absorb or emit energy when revolves
around the nucleus in a fixed orbit.
3. Electron only absorbs or emits energy when jumps
from one orbit to another and the energy absorbed or
released is not continuous but discontinuous in the form
of quantum (photon).
4. Electron releases energy when jumps from higher orbit
to lower orbit but absorbs energy when jumps from lower
orbit to higher orbit.
5. The energy absorbed or released is always equal to the
difference of the energy of the two orbits. ∆E = E2-E1 =
hⱱ
Where ‘h’ is planks constant and ‘ⱱ’ is frequency of
radiation.
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6. Electron only revolves around the nucleus on those
orbits for which the angular momentum (mvr) is equal
to ‘nh/2π’.
mvr = nh/2π
where ‘m’ is mass of electron, ‘v, is velocity, ‘r’ is radius of
orbit, ‘n’ is any natural number (1,2,3-----) and ‘h’ is a plank
constant (6.6262x 10-34J)
Q. 3 (b) Give the electronic configuration of Si(Z= 14) and
Ar (Z= 18)
Ans . Si (Z= 14) 1s2 2s2 2p6 3s2 3p2 so K = 2 L= 8
M= 4
Ar (Z = 18) ) 1s2 2s2 2p6 3s2 3p6 so K = 2 L= 8 M= 8
Q. 4 (a) what are the used of isotopes?
Ans. Uses of Isotopes: Isotopes arc mainly used in chemical,
agricultural and medical research and for diagnosing and
treatment of diseases. For instance:
1. Iodine- 131 is used in the treatment of thyroid cancer.
2. Radium irradiation and cobalt-60 are used in the
treatment of cancer and for diagnosis of tumours.
3. Deuterium, heavy carbon {C-13), heavy nitrogen (N-I 5)
and heavy oxygen (O-18) are being, used as tracer elements
in biochemical and physio-chemical research.
4. Sodium-24 is used for the identification of blood
circulation problems in patients.
Q 4 (b) What do you understand by shielding effect? How it
effect the electron affinity of the element in periodic table?
Ans. Shielding effect: The reduction in the force of nuclear
attraction on the outer electrons due to inner electrons is called
shielding effects or screening effect. OR the reduction in force
of attraction by the shells present between nucleus and
valence electrons is called shielding effect.
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In shielding effect, the core electrons block valence electrons
from the nuclear attraction.
The shielding effects remain constant from left to right in
periods but nuclear charge increases due to which electron
affinity will increase from left to right in periods. The
shielding effects increase from top to bottom in group so
electron affinity decreases from top to bottom in group as
inverse relationship presents between shielding effect and
electron affinity.
Q. 5 (a) Explain Charles law.
Ans. Charles law
Introduction
This law was given by Charles in 1780. He gave a
quantitative relation between volume and absolute
temperature of a gas at constant pressure.
Statement
The volume of a given mass of a gas is directly proportional to
absolute temperature at constant pressure
Mathematical representation
Let the volume of a gas at ‘T’ Kelvin is ‘V’
Then according to Charles’s law
VαT
To convert proportionality into equality a constant is
introduced
V = (constant) T
𝑽
𝑻
= constant
Second statement:
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By using above equation ,Charles’s law can also be stated as:
The ratio of volume to absolute temperature of a gas at
constant pressure is always constant
Similarly
At T1
𝑽𝟏
𝑻𝟏
= constant ---------------(1)
At T2
𝑽𝟐
𝑻𝟐
= constant ---------------(2)
Comparing equation 1 and 2
𝑽𝟏
𝑽𝟐
= 𝑻𝟐
𝑻𝟏
Q. 5 (b) if 3 dm3 of air is heated from 300K to 400K at
constant pressure, then what is the volume of the gas at
higher temperature.
Given Data:
V1 = 3 dm3
T1 = 300K T2= 400K
Required Data; V2= ?
Formula used:
Solution:
𝑽𝟏
𝑽𝟐
= 𝑻𝟐 ( charles law)
𝑻𝟏
V2 =
𝑽𝟏𝑻𝟐
𝑻𝟏
=
𝟑𝑿 𝟒𝟎𝟎
𝟑𝟎𝟎
= 4 dm3
Q. 6 ( a) What is electrolysis? Describe the electrolytic
refining of copper.
Electrolysis
A chemical change caused by the passage of an electric
current through a solution of an electrolyte or molten state is
called "Electrolysis".
OR
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The process of chemical decomposition of an electrolyte with
the help of electric current in fused state or in solution state is
called electrolysis.
Electrolytic refining of copper.
Ans.
Definition: The removal of impurities is called refining while
purification of copper through electrolysis is called electrolytic
refining of copper
Explanation: The copper obtained from their ore usually
contain impurities which are purified by electrolysis. The
electrolytic cell contains anode made from impure copper and
thin cathode made from pure copper. The electrodes are
suspended in a solution of copper sulpahte and sulphuric acid.
When electric current is passed from solution the copper from
impure anode dissolves and form copper ions (Cu+2) which
move towards cathode. When copper ions reach to cathode gain
electrons and convert to pure metallic copper. The impurities
are insoluble in solution so collected under anode.
Q. 6 (b) 5.85 gram of NaCl are dissolved in water to make 50
cm3 solution. Determine its molarity.
Ans.
Given weight of NaCl = 5.85 gram
Molecular weight of NaCl = 23 + 35.5 = 58.5
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𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 𝒊𝒏 𝒈𝒓𝒂𝒎𝒔
Mole of NaCl = 𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆=
𝟓.𝟖𝟓 𝒈𝒓𝒂𝒎
𝟓𝟖.𝟓
= 0.1mole
𝟓𝟎
Volume of solution in dm3 = 𝟏𝟎𝟎𝟎 = 0.05 dm3
𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇𝒎𝒐𝒍𝒆𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆
Molarity = 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝒊𝒏 𝒍𝒊𝒕𝒓𝒆 = 0.1/0.05 = 2 M
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