9th Atoms and molecules NCERT Solution, MCQs, Study Notes, Q & A
1. Q. While searching for
various atomic mass units, scientists initially took 1/16 of the mass an atom
of naturally occurring oxygen as one unit. State two reasons for this.
Ans: initially, scientist chose Hydrogen as a slandered substance to
measure relative mass of element because it it lightest of all elements but latter
oxygen was fix because it is more reactive than hydrogen
Two reasons for this are:
(1)Oxygen reacts with large number
of elements and form compounds
(2)This atomic mass gives masses
of elements in whole number
But latter on it was observed
that atoms of natural occurring oxygen has different masses (called isotopes of
oxygen) than in 1961 IUPAC selected stable isotopes of carbon ( carbon-12) as
the slandered for comparing the atomic mass or molecular mass.
The mass equal to 1/12th of the mass of 126C
atom is called the one atomic mass unit. It is 1.66 × 10-27 kg.
It is represented by symbol 'u' (older symbol 'amu').
1 u = 1/12th
of the mass of carbon-12 atom.
2.Q.What
is relative atomic mass of an element? How it is related to atomic mass
unit?
Ans: The relative atomic
mass (Ar) of an element is the average relative mass of an element
of the element with respect to an atom of Carbon-12 taken as 12u.
Average mass of 1
atom of an element
Relative Atomic mass of an element = --------------------------------------
1/12th of Mass of one C-12 atom
Relative Atomic mass of an element = --------------------------------------
1/12th of Mass of one C-12 atom
The relative atomic mass is a pure number and has no units. It indicates
number of times one atom of an element is heavier than 1/12th of C-12 atom.
Relative atomic mass is also called average atomic mass.
Atomic mass is expressed in amu or u while relative
atomic mass is a number.
Q.3. How
do we know the presence of atoms if they do not exist independently for most of
the elements?
Answer: Atom join in
different way to form matter(neutral molecules or ion) that we are able to
touch, feel and see.
Q.4.The
average atomic mass of a sample of an
element X is 16.2 u. What is the % of isotopes 168X and 18 8X in the sample?
Ans: Let % of
isotopes 168X is
y then 18 8X in
the sample is 100-y
16.2 = y% of 16 + (100-y)%of18
Þ 16.2 x100 = 16y+1800-18y
Þ1620-1800 =-2y
Þ 180=2y
Þy = 90%
% of isotopes
168X is y = 90%
then 18
8X in the sample
is 100-y =100-90=10%
Q.5. (i) Name the body which approves the nomenclature of elements and
compounds.
(ii)
The symbol of sodium is written as Na and not as S. Give reason.
(iii)
Name one element which form diatomic and one which form tetra atomic molecules.
Ans: (a) J. J. Berzelius (b)
The symbol of an element is the
‘first letter’ or the ‘first letter and another letter’ of the English name or
Latin name of the element.
Sodium has Latin name Natrium and so the symbol is Na
(c) oxygen = o2 ozone = o3
Related post to IX(9th) Atoms and Molecules
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