Important  Questions for CBSE Class 9 Science Chapter 3  Atoms and Molecules

IMPORTANT QUESTIONS

1 MARK QUESTIONS
Question. 1. Name the scientist who laid the foundation of chemical sciences. How ?
Answer. Antoine Laurent Lavoisier, by establishing two important laws of chemical combination.

Question. 2. Define law of conservation of mass.
Answer. It states that, ‘Mass is neither created nor destroyed in a chemical reaction.’ In other-words, the mass of the reactants must be equal to the mass of products.

Question. 3. Define law of constant proportion.
Answer. It states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass’.

Question. 4. Which organisation approves the names of elements all over the world ? Write the symbol of gold. [SA II-2014]
Answer.

  • International Union of Pure and Applied Chemistry (IUPAG).
  • Au.

Question. 5. Write the symbols of tungsten and iron.
Answer. (i) Tungsten (W) (ii) Iron (Fe).

Question. 6. Name the element which is used as the reference for atomic mass.
Answer. Carbon.

Question. 7. ‘Atoms of most elements are not able to exist independently’. Name two atoms which exist as independent atoms. [SA II-2012]
Answer. Noble gases such as argon (Ar), helium (He) exist as independent atoms.

Question. 8. What is the number of electrons in Mg atom and { Mg }^{ 2+ } ion ? [SA II-2014]
Answer. Mg:{ 12 }^{ e- }:{ Mg }^{ 2+ }:{ 10 }^{ e- }

Question. 9. Which of the following species is electrically neutral and why ? [SA II-2014]
Answer. { Na }^{ + },{ Cl }^{ - }{ K }^{ + };Na,{ Ca }^{ +2 }

Question. 10. Write atomicity of the following:
(i) Sulphur, (ii) Phosphorus [SA 11-2014]
Answer. (i) Polyatomic, (ii) Tetra atomic.

Question. 11. Define atomicity.
Answer. The number of atoms present in one molecule of an element or a compound is known as its atomicity.

Question. 12. What are polyatomic ions ? Give two examples.
Answer. A group of atoms having a charge is known as polyatomic ion.
Examples : { NH }_{ 4 }^{ 1+ },{ SO }_{ 4 }^{ 2- }

Question. 13. What is the atomicity of Argon ?
Answer. Mono atomic.

Question. 14. Give one relevant reason why scientists choose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit.
Answer. 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. 15. State the number of hydrogen atoms in 1 g of hydrogen.
Answer. One gram of hydrogen = One mole
=6.022 x { 10 }^{ 23 } atoms

Question. 16. What is molar mass ? What are its units ?
Answer. The mass of one mole of a substance is called its molar mass. Its unit is gram per mole (g { mol }^{ -1 }).

Question. 17. Define atomic mass unit.
Answer. One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon -12.

Question. 18. The relative atomic mass of oxygen atom is 16. Explain its meaning.
Answer. The relative atomic mass of an atom is the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Question. 19. Distinguish between molecular mass and molar mass.
Answer. 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. 20. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass ? [SAII-2011]
Answer. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

Question. 21.Which postulate of Dalton’s atomic theory can explain the law of definite proportions ? [SAII-2011]
Answer. Atoms combine in the ratio of small numbers to form compounds. In a compound, the relative number and kinds of atoms are constant.

Question. 22. Define the atomic mass unit. [SAII -2011]
Answer. The mass of 1/12 part of C-12 is equivalent to one atomic mass unit. Previous, it was denoted by symbol amu but now, these days it is denoted by symbol u.

Question. 23. Why is it not possible to see an atom with naked eyes ?
Answer. Because an atom is too small, i.e., the atomic radii of an atom is of the order { 10 }^{ -10 } m to { 10 }^{ -9 } m.

Question. 24. What is meant by the term chemical formula ?
Answer. 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. 25. How many atoms are present in a (i) { H }_{ 2 }S molecule and (ii) { PO }_{ 4 }^{ 3- }ion ? [SAII – 2011]
Answer. (i) In { H }_{ 2 }S, 3 atoms are present (ii) In  { PO }_{ 4 }^{ 3- }, 5 atoms are present.

2 MARKS QUESTIONS
Question. 1. Name the scientists whose  experimentation  established laws of chemical combination. Name the laws also. [SAII-2014]
Answer. Experimentation done by Antoine Laurent Lavoisier and Joseph L Broust established two laws of chemical combination. These laws are :(i) Law of conservation of mass.
(ii) Law of constant proportions.

Question. 2. Give two drawbacks of Dalton’s atomic theory.
Answer. Drawbacks of Dalton’s Atomic Theory :
(i) According to modern theory, atom is not the ultimate indivisible particle of matter. Today, we know that atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.).
(ii) In case of isotopes of an element, the assumption that the atoms of the same element have same mass does not hold good.

Question. 3. Calculate molar mass of sulphuric acid.
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-1

Question. 4. Give the electronic configuration of:
(i) Al atom and its ion
(ii) O atom and its ion
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-2

Question. 5. Give one example each of (i) Monovalent cation (ii) Bivalent cation (iii) Monovalent anion (iv) Bivalent anion
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-3

Question. 6. How would you differentiate between a molecule of element and a molecule of compound ? Write one example of each type. [SA II-2012]
Answer. Molecule of an element is made-up of only one kind of atoms, e.g., { O }_{ 2\quad },{ N }_{ 2 },{ F }_{ 2 },{ O }_{ 3 },{ P }_{ 4 }{ ,S }_{ 8 } etc. Molecule of a compound is made-up of two or more different kinds of atoms in a fixed ratio, e.g., { H }_{ 2 }O,C{ S }_{ 2 },{ H }_{ 2 }S,N{ H }_{ 3 },C{ H }_{ 4 }.

Question. 7. What is meant by a chemical formula ? Give examples. [SA II-2012]
Answer. A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.
e.g., Chemical formula of ammonia is  N{ H }_{ 3 },water is  { H }_{ 2 }O,carbon dioxide is C{ O }_{ 2 }

Question. 8. What is meant by a molecule ? Give examples. [SA II-2012]
Answer. 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 { O }_{ 2 }, ozone is { O }_{ 3 }, phosphorus is { P }_{ 4 }, sulphur is { S }_{ 8 }, etc.

Question. 9. Define one mole, illustrate its relationship with Avogadro constant.
Answer. 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 x { 10 }^{ 23 }. This number is called Avogadro constant or Avogadro number.

Question. 10. 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 ? [SAII -2011]
Answer. 1 g of hydrogen reacts with oxygen = 8 g
3 g of hydrogen reacts with oxygen
= 8 x 3 g = 24 g

Question. 11. Write down the formula of : [SAII-2011]
(i) Sodium oxide (ii) Aluminium chloride
(iii) Sodium sulphide (iv) Magnesium hydroxide
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-4

Question. 12. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen ? Which law of chemical combination will govern your answer ? [SAII -2011]
Answer. 3 g of carbon produce carbon dioxide = 11 g.
The remaining oxygen 50 g – 8 g = 42 g does not take part in the reaction.
The law of definite proportion is governed by the above data.

Question. 13. What are polyatomic ions ? Give examples. [SAII -2011]
Answer. Polyatomic ions: Two or more different atoms unite to form a charged particle is called polyatomic ions.
Examples: {{\left( P{{O}_{4}} \right)}^{3-}} Phosphate, {{\left( N{{O}_{3}} \right)}^{-1}} Nitrate.

Question. 14. Give the names of the elements present in the following compounds:
(a) Quicklime (b) Hydrogen bromide
(c) Baking powder (d) Potassium sulphate.
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-5

Question. 15. What is the mass of: [SAII -2011]
(a) 0.2 mole of oxygen atoms ?
(b) 0.5 mole of water molecules ?
Answer. (a) 1 mole of oxygen atoms = 1 x 16 – 16 g
0.2 mole of oxygen atoms – 16 g x 0.2 = 3.2 g
(b) 1 mole of water ( { H }_{ 2 }O) molecules = 2 x 1 g + 1 x 16 g=18 g
0.5 mole of water ( { H }_{ 2 }O) molecules – 18 g x 0.5 = 9.0 g

3 MARKS QUESTIONS
Question. 1. Write the chemical symbols of two elements:
(i) Which are formed from the first letter of the elements’ name ?
(ii) Whose names have been taken from the names of the elements in Latin ?
(iii) Which are formed from the first two letters of the elements’ name ?
Answer. (i) N (Nitrogen), F(Fluorine), I (Iodine), O (Oxygen) (any two)
(ii) Fe (Ferrum), Cu (Cuprum), Ag (Argentum), Au (Aurum) (any two)
(iii) Ca (Calcium), He (Helium), Al (Aluminium), Si (Silicon) (any two)

Question. 2. Write the correct symbols of the following elements which are written incorrectly. (i) FE (Iron) (ii) AL (Aluminium) (iii) CO (Cobalt) (iv) AG (silver) (v) NA (sodium) (vi) AR (Argon)
Answer. (i) Fe (ii) Al (iii) Co (iv) Ag (v) Na (vi) Ar

Question. 3. State two examples in each case and write their chemical formulae:
(a) Molecules having (me kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-6

Question. 4. State what are ions. Write the formulae of two divalent cations and anions each.
Answer. Ions are charged particles of atoms or group of atoms Cations { Ba }^{ 2+ },{ Mg }^{ 2+ },{ Ca }^{ 2+ }
Anions : { O }^{ 2- },{ S }^{ 2- }

Question. 5. Classify the following compounds diatomic, triatomic and polyatomic molecules: [SAII-2014]
HCl,{ H }_{ 2 },{ H }_{ 2 }O,N{ H }_{ 3 },{ CH }_{ 3 }OH,PC{ l }_{ 5 }
Answer. Diatomic : HCl,{ H }_{ 2 }
Tri atomic : { H }_{ 2 }O
Polyatomic :N{ H }_{ 3 },{ CH }_{ 3 }OH,PC{ l }_{ 5 }

Question. 6. Write the names of the following compounds:
important-question-for-cbse-class-9-science-atoms-and-molecules-7
Answer. (a) Nickel sulphide, (b) Magnesium nitrate,
(c) Sodium sulphate, (d) Aluminium nitrate,
(e) Potassium phosphate, (f) Calcium nitride

Question. 7. Write the chemical names of the following compounds:
important-question-for-cbse-class-9-science-atoms-and-molecules-8
Answer. (a) Potassium sulphate, (b) Magnesium phosphate,
(c) Ammonium chloride, (d) Zinc sulphide,
(e) Sodium nitride, (f) Silver bromide.

Question. 8. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass, sodium carbonate + ethanoic acid —> sodium ethanoate + carbon dioxide + water.
Answer. Mass of reactants before experiment
= Mass of sodium carbonate + Mass of ethanoic acid
= 5.3 g + 6g = 11.3 g
Mass of products after experiment
= Mass of sodium ethanoate + Mass of carbon dioxide + Mass of water
=8.2 g + 2.2 g +0.9 g = 11.3 g
The mass of reactants is equal to the mass of products, therefore, it proves law of conservation of mass.

Question. 9. Write down the names of compounds represented by the following formulae: [SAII -2011]
important-question-for-cbse-class-9-science-atoms-and-molecules-9
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-10
important-question-for-cbse-class-9-science-atoms-and-molecules-11

Question. 10. Calculate the formula unit masses of ZnO, { Na }_{ 2 }O, { K }_{ 2 }C{ O }_{ 3 }, given atomic masses of Zn = 65u, Na = 23u, K = 39 u, C =3 12 u, and O = 16 u. [SAII – 2015]
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-12

Question. 11. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon? [SAII-2011]
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-13

Question. 12. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Answer.
important-question-for-cbse-class-9-science-atoms-and-molecules-14

Question. 13. Write the chemical formulae of the following: [SAII-2015]
(a) Magnesium chloride (b) Calcium oxide
(c) Copper nitrate (d) Aluminium chloride
(e)Calcium carbonate. (f) Ammonium hydroxide
Answer.
important-question-cbse-class-9-science-atoms-molecules-1

Question. 14. What is the mass of:
(a) 1 mole of nitrogen atoms ?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27) ?
(c) 10 moles of sodium sulphite ({ Na }_{ 2 }{ SO }_{ 3 }) ? [SAII-2011]
Answer. (a) 1 mole of nitrogen atoms
= 1 x gram atomic mass of nitrogen atom
= 1 x 14 g = 14 g
(b) 4 moles of aluminium atoms
= 4 x gram atomic mass of aluminium atoms
= 4 x 27 g = 108 g
(c) 10 moles of sodium sulphite ({ Na }_{ 2 }{ SO }_{ 3 })
= 10 (2 x gram atomic mass of Na + 1 x gram atomic mass of sulphur + 3 x- gram atomic mass of oxygen)
= 10(2x23g+1 x32g + 3x16g)
= 10 (46 g + 32 g + 48 g)
= 10 x 126 g = 1260 g

Question. 15. Convert into mole. (a)12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide. [SAII-2011]
Answer.
important-question-cbse-class-9-science-atoms-molecules-2

Question. 16. Calculate the number of molecules of sulphur ({ S }_{ 8 }) present in 16 g of solid sulphur. [SAII – 2011]
Answer.
important-question-cbse-class-9-science-atoms-molecules-3

Question. 17. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u). [SAII-2011]
Answer.
important-question-cbse-class-9-science-atoms-molecules-4
important-question-cbse-class-9-science-atoms-molecules-5

5 MARKS QUESTIONS
Question. 1. Give the postulates of Dalton’s atomic theory.
Answer.

  1. Every element is composed of extremely small particles called atoms.
  2. 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.
  3.  Atoms cannot be created, destroyed or transformed into atoms of other elements.
  4.  Compounds are formed when atoms of different elements combine with each other in small whole number ratios.
  5. The relative number and kinds of atoms in a given compound are constant.

Question. 2. (a) Give one point of difference between an atom and an ion.
(b) Give one example each of a polyatomic cation and an anion.
(c) Identify the correct chemical name of FeS{ O }_{ 3 }: Ferrous sulphate, Ferrous sulphide, Ferrous sulphite.
(d) Write the chemical formula for the chloride of magnesium. [SAII-2014]
Answer.
important-question-cbse-class-9-science-atoms-molecules-6

Question. 3. (a) Define: (i) Molecular mass, (ii) Avogadro constant.
(b) Calculate the number of molecules in 50 g of CaC{ O }_{ 3 }. (Atomic mass of Ca = 40 u, C = 12 u and O = 16 u)
(c) If one mole of sodium atom weighs 23 g, what is the mass (in g) of one atom of sodium ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-7
important-question-cbse-class-9-science-atoms-molecules-8

Question. 4. Calculate the molecular masses of { H }_{ 2 }, { O }_{ 2 }, C{ l }_{ 2 }, C{ O }_{ 2 }, C{ H }_{ 4 }, { C }_{ 2 }{ H }_{ 6 }, { C }_{ 2 }{ H }_{ 4 }, N{ H }_{ 3 }, C{ H }_{ 3 }OH, { H }_{ 2 }S.
Answer.
important-question-cbse-class-9-science-atoms-molecules-9

Question. 5. Calculate the molar mass of the following substances:
(a) Ethyne, { C }_{ 2 }{ H }_{ 2 } (b) Sulphur molecule, { S }_{ 8 }
(c) Phosphorus molecule, { P }_{ 4 } (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCI (e) Nitric acid, HN{ O }_{ 3 } [SAII-2011]
Answer.
important-question-cbse-class-9-science-atoms-molecules-10
important-question-cbse-class-9-science-atoms-molecules-11

APPLICATION BASED QUESTIONS

Question. 1. 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.[SAII-2014]
Answer. 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 proportion.
It states that in a chemical substance, the elements are always present in definite proportions by mass.

Question. 2. State the law of conservation of mass. Is this law applicable to the chemical reactions ? Elaborate your answer with the help of an example. [SAII-2014]
Answer. Law ,of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
Yes, this law is applicable to the chemical reactions.
In all chemical reactions, there is only exchange of reactants taking place when products are formed. There is no loss or gain of mass.
For example, in the following reaction, the total mass of the reactants is equal to the total mass of the products formed.
important-question-cbse-class-9-science-atoms-molecules-12

Question. 3. A sample of vitamin C is known to contain 2.58 x { 10 }^{ 24 } oxygen atoms. How many moles of oxygen atoms are present in the sample ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-13

Question. 4. Sidak took 5 moles of carbon atoms in a container and Sehaj also took 5 moles of sodium atoms in another container of same weight, (a) Whose container is heavier ? (b) Whose container has more number of atoms ?
Answer. (a) Mass of sodium atoms carried by Sehaj = (5 x 23) g = 115 g While, mass of carbon atom carried by Sidak = (5 x 12) g = 60 g Thus, Sehaj’s container is heavy.
(b) Both the bags have same number of atoms as they have same number of moles of atoms.

Question. 5. Fill in the missing data in the table:
important-question-cbse-class-9-science-atoms-molecules-14
Answer.
important-question-cbse-class-9-science-atoms-molecules-15

Question. 6. The visible universe is estimated to contain  { 10 }^{ 22 } stars. How many moles of stars are present in the visible universe ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-16

Question. 7. The formula unit of mass of { Na }_{ 2 }0 is 62 U. Calculate the atomic mass of sodium (Na). (Given atomic mass of oxygen = 16)
Answer.
important-question-cbse-class-9-science-atoms-molecules-17

Higher Order Thinking Skills (HOTS) Questions

Question. 1. Calcium chloride when dissolved in water dissociates into its ions according to the following equation: Ca{ Cl }_{ 2 }\left( aq \right) \longrightarrow { Ca }^{ 2+ }\left( aq \right) +2{ Cl }^{ - }\left( aq \right) Calculate the number of ions obtained from Ca{ Cl }_{ 2 } when 222 g of it is dissolved in water.
Answer.
important-question-cbse-class-9-science-atoms-molecules-18
important-question-cbse-class-9-science-atoms-molecules-19

Question. 2. The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.
Answer. A sodium atom and ion, differ by one electron. For 100 moles each of sodium atoms and ions there would be a difference of 100 moles of electrons.
Mass of 100 moles of electrons = 5.48002 g
important-question-cbse-class-9-science-atoms-molecules-20

Question. 3. Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury is present in 225 g of pure HgS ? Molar mass of Hg and S are 200.6 g  { mol }^{ -1 } and 32 g  { mol }^{ -1 } respectively.
Answer.
important-question-cbse-class-9-science-atoms-molecules-21

Question. 4. The mass of one steel screw is 4.11 g. Find the mass of one mole of these steel screws. Compare this value with the mass of the earth (5.98 x 1024kg). Which one of the two is heavier and by how many times ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-22

Question. 5. Compute the difference in masses of  { 10 }^{ 3 } moles each of magnesium atoms and magnesium ions.
(Mass of an electron = 9.1 x  { 10 }^{ -31 } kg)
Answer.
important-question-cbse-class-9-science-atoms-molecules-23

Question. 6. Which has more number of atoms ? (i) 100 g of { N }_{ 2 } or (ii) 100 g of N{ H }_{ 3 }
Answer.
important-question-cbse-class-9-science-atoms-molecules-24

Question. 7. Compute the number of ions present in 5.85 g of sodium chloride.
Answer.
important-question-cbse-class-9-science-atoms-molecules-25

Question. 8. A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-26

Question. 9. What are ionic and molecular compounds ? Give examples.
Answer. Atoms of different elements join together in definite proportions to form molecules of compounds.
Examples : water, ammonia, carbon dioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation. Examples : sodium chloride, calcium oxide.

Question. 10. Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1 x  { 10 }^{ -28 } g). Which one is heavier ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-27
important-question-cbse-class-9-science-atoms-molecules-28

Question. 11. A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1 % of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.
Answer.
important-question-cbse-class-9-science-atoms-molecules-29

Question. 12. A sample of ethane ({ C }_{ 2 }{ H }_{ 6 }) gas has the same mass as 1.5 x  { 10 }^{ 20 } molecules of methane (C{ H }_{ 4 }). How many { C }_{ 2 }{ H }_{ 6 } molecules does the sample of gas contain ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-30

Question. 13. (a) Calculate the number of molecules of S{ O }_{ 2 } present in 44 g of it.
(b) If one mole of oxygen atoms weighs 16 grams, find the mass of one atom of oxygen in (grams).
Answer.
important-question-cbse-class-9-science-atoms-molecules-31
important-question-cbse-class-9-science-atoms-molecules-32

Question. 14. Sodium is represented as { Na }_{ 11 }^{ 23 }
(а) What is its atomic mass ?
(b) Write its gram atomic mass.
(c) How many atoms of Na will be there in 11.5 g of the sample ?
Answer.
important-question-cbse-class-9-science-atoms-molecules-37

Reasoning Questions

Question. 1. (а) Why does not atomic mass of an element represent the actual mass of its atom ?
(b) “The atomic mass of an element is in fraction.” What does it mean ?
Answer. (a) Atoms of different elements are very small and their actual masses are extremely small. To solve this problem we consider the relative atomic mass of the element. The relative atomic mass of hydrogen is 1 u and its corresponding gram atomic mass is 1 g.
(b) If the atomic mass of an element is in fraction this means that it exists in the form of isotopes. The atomic mass of such element is the average of atomic masses of its isotopes and is generally in fraction.

Question. 2. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mote of Helium gas ?
Answer. 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 monoatomic.

Question. 3. Why atoms form ions ?
Answer. In order to become stable by acquiring the stable electronic configuration of nearest noble gas.

Important Topics/Areas/Questions which are frequently asked in the examination

Question. 1. Write three points of difference between an atom and a molecule. [SAII-2011 ]
Or
What is the difference between an atom and a molecule ? [SAII-2011, 2012]
Answer.
important-question-cbse-class-9-science-atoms-molecules-33

Question. 2. Define formula unit mass. Calculate formula unit mass of NaCI (Atomic mass of Na = 23 u, Cl = 35.5 u)
Answer. 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 NaCI = (23 + 35.5) = 58.5 u.

Question. 3. (a) Give an example in each of the following cases:
(i) a divalent anion. (ii) a trivalent cation.
(iii) a monovalent anion.
(b) Calculate the mass of the following:
(i) 2 moles of carbon dioxide. (ii) 6.022 x  { 10 }^{ 23 } molecules of carbon dioxide.
Answer.
important-question-cbse-class-9-science-atoms-molecules-34

Question. 4. (a) Write the chemical formulae of the following:
(i) Calcium oxide, (ii) Magnesium chloride, (iii) Aluminium hydroxide
(b) Calculate the number of molecules of sulphur ({ S }_{ 8 }) present in 128 g of solid sulphur. (Atomic mass S = 32 u)
Answer.
important-question-cbse-class-9-science-atoms-molecules-35
important-question-cbse-class-9-science-atoms-molecules-36

Important QuestionsNCERT SolutionsRD Sharma Solutions