CBSE Class 9 Chemistry Atoms And Molecules Worksheet

Question. What is meant by the term chemical formula?
Ans : The chemical formula of a compound is a symbolic representation of its composition and actual number of atoms in one molecule of a pure substance may be an atom or a compound

Question. How was the relative atomic mass determined?
Ans : Relative atomic masses were determined by using the laws of chemical combinations and the compound formed.

Question. What is the number of electrons in Mg atom and Mg2+ ion?
Ans : Mg = 12e–
        Mg2+ = 10e–

Question. Define valency.
Ans : The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. Name the instrument which produces image of the surface of element that shows atoms.
Ans : Scanning tunnelling microscope.

Question. The relative atomic mass of oxygen atom is 16. Explain its meaning.
Ans : The relative atomic mass of an atom is the average masses of the atom, as compared to th 12 1 the mass of one carbon-12 atom.

Question. Why atoms form ions?
Ans : Atoms get stability by acquiring the stable electronic configuration of the nearest noble gas for which either they lose electrons or gain electrons and thus acquire noble gas configuration.

Question. Write the postulate given by the Indian philosopher Maharishi Kanad.
Ans : Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles. He said that a time will come when we come across smallest particles beyond which further division will not be possible.

Question. Who was the first scientist to give the concept of formation of compounds?
Ans : Antoine L. Lavoisier gave the concept of formation of compounds.

Question. Define the term gram atom. How is it related to mole and Avogadro number?
Ans : The atomic mass of an element expressed in grams is called gram atomic mass. One gram atom of any element contains 6.022 × 1023 atoms of the element. It is equal to one mole of atoms. One gram atomic mass = 6.022 × 1023 atoms = 1 mole

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

Question. 50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated. Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.
Ans : 50 g of 10% lead nitrate = 5 g lead nitrate
                                                  + 45 g water
50 g of 10% sodium chloride = 5 g sodium chloride
                                                  + 45 g water
Total content before reaction = 5 + 5 + 90 = 100
  Total content after reaction = 90 g
          Amount of precipitate = 6.83 g
According to law of conservation,
Total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100 – 90 – 6.83 = 3.17 g

52. What did Antoine L. Lavoisier observe regarding the formation of compound?
Ans : He noted that many compounds were composed of two or more elements. Each compound had the same elements in the same proportions.

Question. Define one mole, give its relationship with Avogadro constant.
Ans : One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023. This number is called Avogadro constant or Avogadro number.

Question. Who stated the Law of Constant Proportion?
Ans : Joseph Louis Proust stated the Law of Constant Proportion.

Question. Name any two monatomic atoms.
Ans : Sodium, Aluminium.

Question. Write the value of charge of electron.
Ans : 1.6 × 10–19 coulomb.

Question. Name the element which is used as the reference for atomic mass.
Ans : Carbon. 

Question. What are polyatomic ions? Give two examples.
Ans : A group of atoms having a charge is known as polyatomic ion.
        Examples : (NH4)+ (SO4)2–

Question. Explain the law of multiple proportions.
Ans : According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in fixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H2O) and hydrogen peroxide (H2O2) under different condition. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams of oxygen to form hydrogen peroxide. Now, the weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16: 32 or 1 : 2.

Question. What is meant by a molecule? Give examples.
Ans : A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance, e.g., molecule of oxygen is O2, ozone is O3, phosphorus is P4, sulphur is S8, etc.

Question. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Ans : 1 g of hydrogen reacts with oxygen = 8 g 3 g of hydrogen reacts with oxygen = 8 × 3 g= 24 g

Question. Define formula unit mass. Calculate formula unit mass of NaCl (atomic mass of Na = 23u, Cl = 35.5u).
Ans : The formula unit mass is same as molecular mass which is equal to the sum of masses of atoms present in a formula unit. Formula unit mass of NaCl = (23 + 35.5) = 58.5u.

Question. Give two examples of triatomic molecules.
Ans : Carbon dioxide (CO2) and water (H2O)

Question. How does an atom exist?
Ans : Atom exists in the form of atom, molecule or ions.

Question. Define law of constant proportion.
Ans : Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’

Question. State the number of hydrogen atoms in 1 g of hydrogen.
Ans : One gram of hydrogen = One mole = 6.022 × 1023 atoms

Question. State the law of conservation of mass.
Ans : This law states that the mass can neither be created nor destroyed in a chemical reaction. That is ‘bass of reactants is always equal to mass of products.

Question. Who introduced the word ‘Mole’?
Ans : ‘Wilhelm Ostwald’ introduced the word ‘Mole’.

Question. What is Avogadro Constant?
Ans : The number of particles present in one mole of any substance is fixed with a value of 6.022 × 1023.

Question. What is the full form of IUPAC?
Ans : International Union of Pure and Applied Chemistry.

Question. Write atomicity of the following : (i) Sulphur, (ii) Phosphorus
Ans : (i) Polyatomic,
         (ii) Tetra atomic.

Question. What is the difference between an atom and molecule?
Ans : An atom is the smallest particle of an element which may or may not have independent existence. For example : Na, Al, Fe, etc. Molecule is the smallest particle of the element or compound which can exist independently. For example : O2, H2, N2, etc. 25. Name two elements which have same atomic number. Ans : Two elements cannot have the same atomic number.

Question. What is molar mass? What are its units?
Ans : The mass of one mole of a substance is called its molar mass. Its unit is gram per mole (gmol–1).

Question. Give an example in each of the following cases :
(i) a divalent anion
(ii) a trivalent cation
(iii) a mono-valent anion.
Ans : (i) O2– (ii) Fe3+ (iii) I– 

Question. What is the use of mole concept?
Ans : Applications of mole concept :
(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.
(ii) One mole of gas occupies 22.4 litres at 273K.
(iii) One mole of any gas occupies the same volume at same pressure and temperature.
(iv) One mole is equal to 6.022 × 1023 atoms. So, we can calculate the absolute masses of atoms and molecules. 

Question. Give the postulates of Dalton’s atomic theory.Ans : Every element is composed of extremely small particles called atoms. Atoms of a given element are identical, both in mass and properties. Different chemical elements have different kinds of atoms; in particular, their atoms have different masses. Atoms cannot be created, destroyed or transformed into atoms of other elements. Compounds are formed when atoms of different elements combine with each 115. 

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.

Question. When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of chemical combination will govern your answer? State the law.
Ans : When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. It means magnesium and oxygen are combined in the ratio of 3 : 2 to form magnesium oxide.
Thus, when 3.00 g of magnesium is burnt in 5.00 g of oxygen, 5.00 g of magnesium oxide will be formed and the remaining oxygen will be left unused. It is governed by law of definite proportions.
It states that in a chemical substance, the elements are always present in definite proportions by mass.

Question. Calculate the following in 5.6 g of nitrogen :
(a) Number of moles of nitrogen
(b) Number of molecules of nitrogen
(c) Number of atoms of nitrogen
Ans : (a) Molar mass of nitrogen = 14 g
                     5.6 g of nitrogen = . 14 = 5 6 = 0.4 mole
         (b) 28 g of nitrogen = 6.022 × 1023 molecules
              5.6 g of nitrogen = 2.15 × 1022 molecules
         (c) 14 g of nitrogen = 6.023 × 1023 atoms
              5.6 g of nitrogen = 4.30 × 1023 atoms

Question. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Ans : N (Nitrogen),   F (Fluorine),   I (Iodine),    O (Oxygen) (any two).

Question. What is the atomicity of argon?
Ans : Mono atomic.

Question.. Distinguish between molecular mass and molar mass.
Ans : The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.

Question. Give the symbol of copper, silver, gold, oxygen, zinc.
Ans : Copper –
         Cu Silver – Ag
         Gold – Au
         Oxygen – O
         Zinc – Zn

Question. What is the building block of all matter?
Ans : Atom is the building block of all matter.

Question. What is the measuring unit of atomic radius?
Ans : Nanometre (nm) is the measuring unit of atomic radius.

Question. What is the difference between hydrogen chloride and nitrogen molecule formation?
Ans : Hydrogen chloride is molecular compound and formed by the union of different kinds of atoms while nitrogen is diatomic molecule and formed by union of two atoms of same kinds.

Question. Name two atoms which exist as independent atoms.
Ans : Noble gases such as argon (Ar) and helium (He) exist as independent atoms.

1M

1. The atomicity of noble gases is
(A) Mono-atomic
(B) Octa-atomic
(C) Tri-atomic
(D) Tetra-atomic

2. Write the formula of one polyatomic cation alongwith its valency. .

3. What is the mass of carbon dioxide released in the reaction ?

4. Calculate the number of molecules of carbon dioxide present in 4-4 g of it.

5. Law of Constant Proportions was given by
(A) Newton
(B) Rutherford
(C) Lavoisier
(D) Proust

6. 5g of Calcium (Ca) contains [Atomic mass of Ca = 40u]
(A) 1/8mol
(B) 1/7 mol
(C) 1/8g/mol
(D) 1/9mol

7. 1 mole of NH₃ molecules contains
(A) 6.022X10²³NH₃ molecules
(B) 6.022X10²² NH₃ atoms
(C) 6.022X10²³ NH₃ atoms
(D) 6.022X10²² NH₃ molecules

8. Unit of molar mass is
(A) Gm/Molecule
(B) Gm
(C) Moles
(D) gm/mol

9. Valencies of iron are
(A) 2 and 3
(B) 1 and 3
(C) 1, 2 and 3
(D) 1 and 2

10. Cuprous sulphide is the chemical name of
(A) CuS
(B) Cu₂S
(C) CuS₂
(D) Cu₃S

11. Mass of 0.5 M of oxygen atom is:
(A) 16 g
(B) 32 g
(C) 4 g
(D) 8 g

12. An ionic compound is made up of
(A) Metal-nonmetal
(B) Cation-Cation
(C) Anion-Anion
(D) None of the above

13. Write the chemical formula of "Aluminium phosphate".

14. Atom consists of electron, proton and neutron, which means it is divisible. This statement
(A) Strengthens Dalton's Atomic Theory
(B) Weakens Dalton's Atomic Theory
(C) Has no effect on Dalton's Atomic Theory
(D) None of the above

15. When 100g of CaCO₃ was heated, it decomposes to yield 56g of CaO and 44g of CO₂ . This statement proves that
(A) Mass is conserved during a chemical reaction
(B) There is complete loss of CaCO₃ during a chemical reaction
(C) Mass of reactant is different from mass of product
(D) CaCO₃ decomposes into Ca and CO₂

16. Write down the formulae of aluminium sulphate and ethanol.

17. Which are called the building blocks of matter?

18. Write the chemical symbols of -
(c) Tin
(d) Mercury

19. How many metres are there in 1 nanometer (nm) ?

20. Define atomicity.

21. Atomicities of phosphorus and ozone are respectively:
(A) 4, 3
(B) 2, 2
(C) 2, 3
(D) 4, 2

22. What do we call those particles which have-
(a) Less electrons than the normal atoms.
(b) More electrons than the normal atoms.

23. Cation is formed due to
(A) Gain of electrons
(B) Loss of protons
(C) Loss of electrons
(D) Gain of protons

24. Match the following molar masses and choose the answer from the code given below
              Column I                         Column II
(i) Sulphuric acid H₂SO₄          (A) 78g/mol
(ii) Benzene C₆H₆                    (B) 164g/mol
(iii) Sodium Chloride NaCl       (C) 98g/mol
(iv) Calcium nitrate                   (d) 58.5g/mol
[Atomic mass of: Ca = 40u, H = 1u, S = 32u, C = 12u, Na = 23u, N = 14u, O = 16u, Cl = 35.5u]
(A) (i) -C, (ii) -A, (iii) -D, (iv) -B
(B) (i) -B, (ii) -A, (iii) -D, (iv) -C
(C) (i) -C, (ii) -A, (iii) -B, (iv) -D
(D) (i) -A, (ii) -D, (iii) -C, (iv) -B

25. 144 grams of pure water is decomposed by passing electricity. 16 grams of hydrogen and 128 grams of oxygen are obtained. Which chemical law is illustrated by this statement?

26. What is the atomicity of -
(a) Ozone
(b) Nitrogen

27. What is the atomicity of -
(a) Neon
(b) Sulphur

28. What name is given to the number 6.023 x 10²³?

29. Select the correct statement and choose the answer from the code given below-
I. Gram molecular mass of a substance contains 6.022X1023molecules.
II. The molar mass of a substance is the mass of 1 mole of that substance.
III. Gram molecular mass of H₂O is 1.
IV. The molar mass of nitrogen molecule is 28g/mol
(A) I and III
(B) I, II and IV
(C) Only II
(D) All of these

30. Which of the following are ionic compounds?
(i) NH₄+ (ii) CaCl₂ (iii) C₆H₆ (iv) Al₂O₃
(A) (i) and (ii)
(B) (ii) and (iv)
(C) (i) and (iii)
(D) None of the above

31. Select the correct statement and choose the answer from the code given below
(i) All metals form cations by gaining electrons
(ii) All metals form cations by losing electrons
(iii) Hydrogen is a non metal but forms a cation
(iv) All nonmetals form anions
(A) (ii) only
(B) (iii) and (iv)
(C) (i) and (iii)
(D) (ii) , (iii) and (iv)

32. An atom is made up of negatively charged electrons and positively charged protons, still it is electrically neutral. The reason is
(A) The electrons and protons are equal in number so no net charge is present.
(B) The neutrons neutralize the charge and make the atom neutral
(C) The charge on the atom is too small to be detected
(D) Both (1) and (2)

33. When calcium metal (Ca) changes to calcium ion (Ca⁺₂) the number of electron, proton andneutron
(A) Remains same
(B) Number of neutrons remains the same while the number of both protons and electrons changes.
(C) Number of neutrons and protons remains same while the number of electrons decreases by 2.
(D) Number of neutrons and electrons remains the same while number of protons increases by 2

34. What is the contradiction against Dalton's atomic theory in the formula of sucrose, C^22^u ?

35. Select the correct statement and choose the answer from the code given below

I.One mole of all substances in gaseous state contains equal number of particles (6.022X10²³) and occupy equal volume (22.4 L) at STP.
II.One mole of all substances contains equal number of particles (6.022X10²³) at all temperatures and pressure.
III.Avogadro number is equal to 6.022X10²³ particles (atoms, molecules,ions) .
IV.One mole of N₂ does not contain same number of molecules as in one mole of H₂.
(A) I and IV
(B) I and III
(C) I, III and IV
(D) None of these

36. Write the postulate of Dalton's atomic theory that eplains the Law of definite proportions.

37. The number of atoms present in 0.4 mole of Calcium (Ca) are
(A) 2.408 X 10²³
(B) 24.08 X 10²³
(C) 2.408 X 10²⁴
(D) 6.022 X 10²³

38. An element A forms an oxide A₂O₃, the valency of A is
(A) 2
(B) 3
(C) 5
(D) 1

39. Formula mass of ammonium sulphate (NH₄) 2SO₄ is (N=14, H=1, O=16 and S=32)
(A) 114u
(B) 132u
(C) 120u
(D) 132u

40. The chemical formula of Phosphorus and Sulphur are respectively:
(A) P4 and S
(B) P3 and S8
(C) P4 and S8
(D) P4 and S6

41. Which of the following gases can exist in atomic form?
(A) Oxygen (O)
(B) Neon (Ne)
(C) Hydrogen (H)
(D) Nitrogen(N)

42. Define 'Atomic mass unit' or what is '1 «' ?

43. Write the formula of 'Limestone'. Name the elements present in it.

44. What is the molar mass of a sulphur molecule ?

45. What is Latin name of 'silver' ? What is its symbol ?

46. Write the chemical symbols of -
(a) Silver
(b) Potassium

47. Match the correct formula and choose the answer from the code given below
(i) Hydrogen sulphide              (A) CuO
(ii) Ammonia                            (B) Al₂O₃
(iii) Aluminium oxide                (C) H₂S
(iv) Cupric Oxide                     (D) NH₃
(A) (i) -C,(ii) -A,(iii) -D,(iv) -B
(B) (i) -A, (ii) -D, (iii) - C, (iv) -B
(C) (i) -C, (ii) -D, (iii) -B, (iv) -A
(D) (i) -B, (ii) -D, (iii) -A, (iv) -C

48. Calculate the mass of one amu in kilograms. .

49. Ratio of nitrogen and hydrogen in ammonia (NH3) by mass is:
(A) 3 : 1
(B) 1 : 3
(C) 3 :14
(D) 14 : 3

50. Which of the following statement is correct? Choose the answer from the code given below.
I Dalton proposed the term Atom
II.Maharishi Kanad proposed the term Atom
III.Atom was considered to be indivisible by Dalton
IV. Kanad said Atom could be further divided
(A) I only
(B) II and IV
(C) I and III
(D) All of these