CBSE Class 9 Chemistry Atoms And Molecules Worksheet Set D

Question. Give one relevant reason, why scientists choose th 16 1 of the mass of an atom of naturally occurring oxygen as the atomic mass unit?
Ans : Initially, 1/16th of the mass of naturally occurring oxygen was taken as the atomic mass unit because this unit gave masses of most of the elements as whole numbers.

Question. Explain the form of atoms in a solid.
Ans : A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these. They are different in physical and chemical properties.

Question. Give two drawbacks of Dalton’s atomic theory.
Ans : Drawbacks of Dalton’s atomic theory :
(i) According to modern theory, atom is not the ultimate indivisible particle of matter. Atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.).
(ii) The assumption that the atoms of the same element have same mass does not hold good, in case of isotopes of an element.

Question. Calculate the mass of the following :
(i) 2 moles of carbon dioxide.
(ii) 6.022 × 1023 molecules of carbon dioxide. 
Ans : (i) Molar mass of CO2 = 12 + 2 × 16 g = 44 g
1 mole of carbon dioxide = 44 g
2 mole of carbon dioxide = 44 g × 2 = 88 g
(ii) Molar mass of CO2 = 44 g
         = 6.022×1023 molecules of carbon dioxide

Question. State two examples in each case and write their chemical formulae :
(a) Molecules having same kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Ans : (a) F2, Cl2, P4, S8
         (b) Ammonia (NH3), Sulphur dioxide (SO2), Carbon disulphide (CS2).
         (c) Calcium sulphate (CaSO4), Sodium nitrate (NaNO3).

Question. Find the number of atoms in the 0.5 mole of C atom.
Ans : 0.5 mole of C atom :
Number of atoms in 1 mole of C atom
                              = 6.022 × 1023 atoms
Number of atoms in 0.5 mole of C atom
                             = 6.022 × 1023 × 0.5
                             = 3.011 × 1023 atoms

Question. What are molecules? Give brief explanation of the arrangement of the constituent atoms in the molecules.
Ans : A molecule is the smallest particle of an element or compound which is stable in normal conditions. And it can freely show all the properties of that element or compound. It may be made up of one, two or more atoms. Molecule with one atom called monoatomic. E.g. helium, neon, etc. Molecule with two atoms called diatomic. E.g.
Cl2, O2. Similarly, there are molecules containing three atoms (CO2), four atoms (P4) and so on.

Question. What is the ratio by mass of combining elements in H2O, CO2 and NH3 ?
Ans : H2O ratio by mass of combining elements
                                           = 2 : 16 = 1 : 8 (H : O)
CO2 ratio by mass of combining elements
                                           = 12 : 32 = 3 : 8 (C : O)
NH3 ratio by mass of combining elements
                                           = 14 : 3 = 14 : 3 (N : H)

Question. Write the name of the compounds : NaBr, Al2O3, CaCO3.
Ans : NaBr = Sodium bromide
        Al2O3 = Aluminium oxide
       CaCO3 = Calcium carbonate

Question. Why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas? Explain.
Ans : The number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas because hydrogen molecule is diatomic, i.e., a molecule of hydrogen consists of two atoms of hydrogen, whereas helium is monatomic.

Question. Name the gas which gives the lightest positively charged particle.
Ans : Hydrogen.

Question. What do we get if 3 atoms of oxygen unite into a molecule, instead of usual 2 ?
Ans : We get (O3) ozone.

Question. Define atomicity.
Ans : The number of atoms present in one molecule of an element or a compound is known as its atomicity.

Question. Give difference between 2H and H2.
Ans : 2H indicates 2 atoms of hydrogen and H2 indicates one molecule of hydrogen.

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