# TNPSC - Atoms and Molecules - 10th Standard Science

1.      From the given examples, form the pair of isotopes and the pair of isobars: 18Ar40, 17CI35, 20Ca40, 17CI37
Answer:  Isotopes     :     17CI35, 17CI37
Isobars       :      18Ar40, 20Ca40

2.       Molecular mass of Nitrogen is 28.  Its atomic mass is 14.  Find the atomicity of Nitrogen.   (SEP - 2014, APRIL – 2014, 12)
Answer:  Atomicity   =   Molecular Mass
Atomic Mass
=     28
14
Atomicity             =      2

3.      Gram molecular mass of oxygen is 32g/  Density of Oxygen is 1.42g/cc.  Find the gram molecular volume of Oxygen.
Gram Molecular Volume    =    Gram Molar Mass
Density
=         32
1.429
Gram Molecular Volume of Oxygen     =    22.4 litre

4.       ‘Cl’ represents chlorine atom, ‘CI₂’ represents chlorine molecule.   List our any two difference between atoms and molecules.
 Atom Molecule 1.  Atom is an non bonded entity. 1.  Molecule is a bonded entity. 2.  It may or may not exist freely. Eg: CI 2.  It can exist freely.   Eg: CI₂

5.      Calculate the gram molecular mass of water from the values of gram atomic mass of hydrogen and of Oxygen.  (APRIL – 2013,12)
Gram atomic mass of hydrogen   =   1g
Gram atomic mass of oxygen       =   16g
Answer:   H₂O =  2[H] + 1[O]
Gram molecular mass of water   = 2[1] + 1[16]  =  2 + 16
Gram molecular mass of water  =  18g

6.       One mole of any substance contains 6.023 ×1023 particles.
If 3.0115  ×  1023 particles are present in CO₂.  Find the number of moles.
Number of moles  =  No. Of molecules
6.023 × 1023
=   3.0115 × 1023
6.023 ×1023
=  0.5 moles

7.      ________ have equal number of neutrons.
a. Isobars
b. Isotones
c. Isotopes
d. Mass Numbers
Answer:  b. Isotones

8.     Classify the following based on atomicity:
a. Chlorine
b. Neon
c. Phosphorous
d. Ozone
Answer:    Chlorine             -       Diatomic Molecule
Neon                  -       Monotonic Molecule
Phosphorous      -       Polyatomic Molecule
Ozone                -       Triatomic Molecule

9.      Identify and correct the mistake in each of the following:
a. The molar volume of gas at STP is 22.4 cm₃.
b. 2 × R.M.M. = V.D
c. An atom cannot exist independently.
d. The ratio of atoms in a molecule may be integral or simple or may not be fixed.
d. H₂O is a homo atomic molecule.
Answer:  a. The molar volume of gas at STP is 22.4 Litre.
b. 2 × V.D. = R.M.M.
c. An atom may or may not exist independently.
d. The ratio of atoms in a molecule are intergral and  fixed.
d. H₂O  is a  Hetero atomic molecule.